People buying bitcoin should be prepared to lose their money, according to a senior UK regulator.
The price of the cryptocurrency has been soaring in recent weeks. With that has come many new investors, hoping to make money from the surging interest in the virtual currency.
But others have warned that the rocketing value of bitcoin can only be temporary, and that it is stuck in a bubble. That could eventually come to an end in a spectacular burst, warned Andrew Bailey, head of the Financial Conduct Authority.
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1/43 Designed by Pierpaolo Lazzarini from Italian company Jet Capsule. The I.F.O. is fuelled by eight electric engines, which is able to push the flying object to an estimated top speed of about 120mph. Jet Capsule/Cover Images
2/43 A humanoid robot gestures during a demo at a stall in the Indian Machine Tools Expo, IMTEX/Tooltech 2017 held in Bangalore Getty Images
3/43 A humanoid robot gestures during a demo at a stall in the Indian Machine Tools Expo, IMTEX/Tooltech 2017 held in Bangalore Getty Images
4/43 Engineers test a four-metre-tall humanoid manned robot dubbed Method-2 in a lab of the Hankook Mirae Technology in Gunpo, south of Seoul, South Korea Jung Yeon-Je/AFP/Getty Images
5/43 Engineers test a four-metre-tall humanoid manned robot dubbed Method-2 in a lab of the Hankook Mirae Technology in Gunpo, south of Seoul, South Korea Jung Yeon-Je/AFP/Getty Images
6/43 The giant human-like robot bears a striking resemblance to the military robots starring in the movie 'Avatar' and is claimed as a world first by its creators from a South Korean robotic company Jung Yeon-Je/AFP/Getty Images
7/43 Engineers test a four-metre-tall humanoid manned robot dubbed Method-2 in a lab of the Hankook Mirae Technology in Gunpo, south of Seoul, South Korea Jung Yeon-Je/AFP/Getty Images
8/43 Waseda University's saxophonist robot WAS-5, developed by professor Atsuo Takanishi Rex
9/43 Waseda University's saxophonist robot WAS-5, developed by professor Atsuo Takanishi and Kaptain Rock playing one string light saber guitar perform jam session Rex
10/43 A test line of a new energy suspension railway resembling the giant panda is seen in Chengdu, Sichuan Province, China Reuters
11/43 A test line of a new energy suspension railway, resembling a giant panda, is seen in Chengdu, Sichuan Province, China Reuters
12/43 A concept car by Trumpchi from GAC Group is shown at the International Automobile Exhibition in Guangzhou, China Rex
13/43 A Mirai fuel cell vehicle by Toyota is displayed at the International Automobile Exhibition in Guangzhou, China Reuters
14/43 A visitor tries a Nissan VR experience at the International Automobile Exhibition in Guangzhou, China Reuters
15/43 A man looks at an exhibit entitled 'Mimus' a giant industrial robot which has been reprogrammed to interact with humans during a photocall at the new Design Museum in South Kensington, London Getty
16/43 A new Israeli Da-Vinci unmanned aerial vehicle manufactured by Elbit Systems is displayed during the 4th International conference on Home Land Security and Cyber in the Israeli coastal city of Tel Aviv Getty
17/43 Electrification Guru Dr. Wolfgang Ziebart talks about the electric Jaguar I-PACE concept SUV before it was unveiled before the Los Angeles Auto Show in Los Angeles, California, U.S Reuters
18/43 The Jaguar I-PACE Concept car is the start of a new era for Jaguar. This is a production preview of the Jaguar I-PACE, which will be revealed next year and on the road in 2018 AP
19/43 Japan's On-Art Corp's CEO Kazuya Kanemaru poses with his company's eight metre tall dinosaur-shaped mechanical suit robot 'TRX03' and other robots during a demonstration in Tokyo, Japan Reuters
20/43 Japan's On-Art Corp's eight metre tall dinosaur-shaped mechanical suit robot 'TRX03' Reuters
21/43 Japan's On-Art Corp's eight metre tall dinosaur-shaped mechanical suit robot 'TRX03' performs during its unveiling in Tokyo, Japan Reuters
22/43 Singulato Motors co-founder and CEO Shen Haiyin poses in his company's concept car Tigercar P0 at a workshop in Beijing, China Reuters
23/43 The interior of Singulato Motors' concept car Tigercar P0 at a workshop in Beijing, China Reuters
24/43 Singulato Motors' concept car Tigercar P0 Reuters
25/43 A picture shows Singulato Motors' concept car Tigercar P0 at a workshop in Beijing, China Reuters
26/43 Connected company president Shigeki Tomoyama addresses a press briefing as he elaborates on Toyota's "connected strategy" in Tokyo. The Connected company is a part of seven Toyota in-house companies that was created in April 2016 Getty
27/43 A Toyota Motors employee demonstrates a smartphone app with the company's pocket plug-in hybrid (PHV) service on the cockpit of the latest Prius hybrid vehicle during Toyota's "connected strategy" press briefing in Tokyo Getty
28/43 An exhibitor charges the battery cells of AnyWalker, an ultra-mobile chasis robot which is able to move in any kind of environment during Singapore International Robo Expo Getty
29/43 A robot with a touch-screen information apps stroll down the pavillon at the Singapore International Robo Expo Getty
30/43 An exhibitor demonstrates the AnyWalker, an ultra-mobile chasis robot which is able to move in any kind of environment during Singapore International Robo Expo Getty
31/43 Robotic fishes swim in a water glass tank displayed at the Korea pavillon during Singapore International Robo Expo Getty
32/43 An employee shows a Samsung Electronics' Gear S3 Classic during Korea Electronics Show 2016 in Seoul, South Korea Reuters
33/43 Visitors experience Samsung Electronics' Gear VR during the Korea Electronics Grand Fair at an exhibition hall in Seoul, South Korea Getty
34/43 Amy Rimmer, Research Engineer at Jaguar Land Rover, demonstrates the car manufacturer's Advanced Highway Assist in a Range Rover, which drives the vehicle, overtakes and can detect vehicles in the blind spot, during the first demonstrations of the UK Autodrive Project at HORIBA MIRA Proving Ground in Nuneaton, Warwickshire PA wire
35/43 Chris Burbridge, Autonomous Driving Software Engineer for Tata Motors European Technical Centre, demonstrates the car manufacturer's GLOSA V2X functionality, which is connected to the traffic lights and shares information with the driver, during the first demonstrations of the UK Autodrive Project at HORIBA MIRA Proving Ground in Nuneaton, Warwickshire PA wire
36/43 Ford EEBL Emergency Electronic Brake Lights is demonstrated during the first demonstrations of the UK Autodrive Project at HORIBA MIRA Proving Ground in Nuneaton, Warwickshire PA
37/43 Full-scale model of 'Kibo' on display at the Space Dome exhibition hall of the Japan Aerospace Exploration Agency (JAXA) Tsukuba Space Center, in Tsukuba, north-east of Tokyo, Japan EPA
38/43 Miniatures on display at the Space Dome exhibition hall of the Japan Aerospace Exploration Agency (JAXA) Tsukuba Space Center, in Tsukuba, north-east of Tokyo, Japan. In its facilities, JAXA develop satellites and analyse their observation data, train astronauts for utilization in the Japanese Experiment Module 'Kibo' of the International Space Station (ISS) and develop launch vehicles EPA
39/43 The robot developed by Seed Solutions sings and dances to the music during the Japan Robot Week 2016 at Tokyo Big Sight. At this biennial event, the participating companies exhibit their latest service robotic technologies and components Getty
40/43 The robot developed by Seed Solutions sings and dances to music during the Japan Robot Week 2016 at Tokyo Big Sight Getty
41/43 Government and industry are working together on a robot-like autopilot system that could eliminate the need for a second human pilot in the cockpit AP
42/43 Aurora Flight Sciences' technicians work on an Aircrew Labor In-Cockpit Automantion System (ALIAS) device in the firm's Centaur aircraft at Manassas Airport in Manassas, Va. AP
43/43 Flight Simulator Stefan Schwart and Udo Klingenberg preparing a self-built flight simulator to land at Hong Kong airport, from Rostock, Germany EPA
He has given a "serious warning" that people investing in the cryptocurrency should be ready to lose their money.
We know "relatively little" about how the prices change and so we should expect it to stay volatile as it has been over recent months, he cautioned. But that volatility could eventually be the result of a dramatic plunge, which would leave many people invested in the cryptocurrency out of pocket.
"It's not a currency, it's actually not regulated in its Bitcoin form," Mr Bailey said in an interview for Newsnight. "It's a very volatile commodity in terms of its pricing.
"If you look at what has happened this year, I would caution people. We know relatively little about what informs the price of Bitcoin.
"It's an odd commodity as well, as the supply is fixed. If you want to invest in Bitcoin be prepared to lose your money – that would be my serious warning."
Many have criticised regulators including Mr Bailey's organisation for failing to keep up with bitcoin, and allowing it to be used with little restriction. The cryptocurrency has been accused of allowing criminals to flourish since it can be used anonymously and without much oversight, allowing it to flourish as a way of buying drugs and guns on the dark net, for instance.
But he said that since it is a commodity and not really a currency, it didn't need to be regulated in the same way and any new powers would be up to parliament to decide.
Wechselkurs zwischen US-Dollar und Bitcoins bis 2019
 Felix Hufeld, Präsident der Bundesanstalt für Finanzdienstleistungsaufsicht (BaFin), hat Geldanleger vor der internationalen Bitcoin-Währung gewarnt. Es bestehe die Gefahr des Totalverlustes durch Kurs-Spekulationen. Vor Weihnachten war der Kurs innerhalb weniger Tage um fast 50 % gefallen, nachdem ein zunehmendes Interesse an der Börse zunächst für einen Anstieg seit Anfang des Jahres von 1.000 bis auf 20.000 Euro gesorgt hatte. Der Kursverlauf ist jedoch nicht ungewöhnlich: wenn viele Anleger wieder verkaufen, um Gewinne zu erzielen, kann der Preis schnell wieder fallen. 

Bereits am 16. Dezember 2017 hatte auch der Chef der britischen Finanzaufsichtsbehörde auf das Risiko hingewiesen. Hinzu kommt, dass es zahlreiche derivative Finanzinstrumente gibt, die zusätzlich das Marktgeschehen beeinflussen. Dabei werden nicht die Bitcoins selbst gekauft, sondern oft nur Wetten auf den Preisverlauf abgeschlossen. Mit der realen Wirtschaft hat das zwar nichts zu tun, doch wird durch die dezentrale Verwaltung der Bitcoins in einem weltweiten Rechnernetzwerk und die exponentiell steigenden Rechenoperationen im Verhältnis sehr viel Energie verbraucht. Hochrechnungen haben ergeben, dass durch die Zunahme der Währungsspekulation und des damit verbundenen höheren Kohlendioxidausstoßes negative Auswirkungen auf das globale Klima zu befürchten sind. Somit haben Bitcoin-Spekulationen letztlich größere Auswirkungen als ursprünglich angenommen, und Klimaschützer warnen vor den Folgen.

Berichten zufolge spielt die Innere Mongolei dabei eine besondere Rolle. Hier gibt es zahlreiche Kohlekraftwerke, die mit billigem Strom die Computer der chinesischen Spekulanten beliefern. Einer Berechnung zufolge erzeugt jeder Bitcoin in der Inneren Mongolei 8 bis 13 Tonnen CO2.

Gibt es eine Gefahr für das Weltklima durch Bitcoins?