Police have been searching the area around the car bomb finds
Two Mercedes - left on Friday outside a club in Haymarket and a nearby street - contained petrol, gas cylinders and nails but the devices did not detonate.
On Saturday a blazing car was driven into Glasgow Airport's terminal building. Police said it was too early to say if it was linked.
Police in London are studying CCTV footage in the search for suspects.
'Worrying similiarities'
Prime Minister Gordon Brown is expected to chair the third meeting in two days of the government emergencies committee Cobra later.
This comes after police in Scotland confirmed a burning car had been driven into the main terminal building at Glasgow Airport after 1515BST. Two people have since been arrested.
Mr Brown has spoken to Scotland's First Minister Alex Salmond and Home Secretary Jacqui Smith.
But Downing Street has refused to comment on whether the incidents in Scotland and London are linked, saying this was a matter for the police.
The BBC's Andy Tighe said there were "worrying similarities" between the two incidents but no-one in authority had yet made any "firm linkage."
Earlier Cobra met to discuss the London car bomb attempts. Ms Smith said after this meeting the threat must not stop people getting on with their lives.
Both Mercedes were found to contain explosive materials
Car bomb leads Eyewitness accounts
Ms Smith: "The prime minister stressed all parts of government must work together to combat the terror threat."
She urged the public to report anything suspicious.
CCTV footage is being studied, with unconfirmed reports suggesting police may have an image of a suspect leaving the vehicle left outside the Tiger Tiger club in Haymarket, near Piccadilly Circus.
Security reviewed
Police have increased patrols and security for events in the city over the weekend, including the Gay Pride parade, the Concert for Diana at Wembley Stadium and the Wimbledon tennis championships.
TIGER TIGER NIGHTCLUB Thursday night was 'Ladies Night' Maximum capacity: 1,700 on four floors Witnesses said there were about 500 people there on Thursday
Met Police Assistant Commissioner Tarique Ghaffur said a "very significant security operation" was in place.
"What is really amazing is that London wants to - and has - returned to normality and lots of events that are supposed to take place are taking place," he said.
Petrol vapour
News that the second Mercedes also contained explosive materials came on Friday evening, several hours after details of the car abandoned outside Tiger Tiger had emerged.
Simultaneous, co-ordinated bombs have historically been a distinguishing feature of [al-Qaeda's] anti-Western attacks
Frank Gardner
BBC security correspondent
Car bombs timeline In pictures: Car bomb
BBC security correspondent Frank Gardner said simultaneous, co-ordinated bombs have been historically a distinguishing feature of al-Qaeda attacks.
The first bomb was found when an ambulance crew, called to Tiger Tiger at 0125 BST to deal with someone taken ill, spotted what appeared to be vapour inside a metallic green Mercedes parked outside the club.
The second device was discovered in a blue Mercedes 280E at a Park Lane car pound after the smell of petrol was detected.
The vehicle had been towed away at 0330 BST on Friday after being given a ticket for illegally parking in Cockspur Street, near Trafalgar Square.
Both contained bombs which were similar, potentially viable and clearly linked, police said.
Previous cases
It is understood the devices were intended to have been detonated remotely by mobile phone but for some reason they did not work.
Police are looking at footage from CCTV close the parked cars
Deputy Assistant Commissioner Peter Clarke, head of Scotland Yard's counter-terrorism command, said the discovery of the second device was "obviously troubling" and "reinforces the need for the public to be alert".
He said the Haymarket incident "resonated" with previous cases.
Five men were jailed for life in April for a bomb plot linked to al-Qaeda that planned to use a fertiliser bomb to target a shopping centre and a nightclub.
And Dhiren Barot was jailed for life last November for conspiring to park limousines packed with gas canisters underneath high-profile buildings before detonating them.
The current terror threat level has been classed severe - one level lower than the highest "critical" - since 14 August 2006.
Police have urged anyone with information to phone the confidential Anti-Terrorist hotline number on 0800789321.
Mercedens-Benz, w którym znajdowała się bomba
Klub nocny, który miał być celem pierwszej bomby 
Wg agencji AFP w Londynie znaleziono "podejrzany samochód" , który może okazać trzecim dziś znalezionym ładunkiem wybuchowym w stolicy Wielkiej Brytanii. 

Jak dowiedziała sie CNN w obu samochodach zidentyfikowanych jako samochody-pułapki znaleziono części składowe bomb. Wcześniej Scotland Yard informował o tym że w samochodach znajdowały się Butle z gazem, benzyna i mnóstwo gwoździ.  

"Brytyjska policja rozbroiła w piątek rano w centralnym Londynie domniemaną bombę" - podano w policyjnym komunikacie.

Na jednej z z głównych ulic Londynu - Haymarket - w samochodzie znaleziono "potencjalnie groźne urządzenie wybuchowe, które zostało rozbrojone" - podano w komunikacie, opublikowanym w Londynie.

Grupa policyjna ds. zwalczania terroryzmu prowadzi dochodzenie w tej sprawie.

Drugą bombę znaleziono również w zaparkowanym samochodzie niedaleko Pałacu Buckingham.
Policja zamknęła ruchliwe ulice Park Lane i Oxford Street w centrum miasta.

Policja poinformował, że nie jest w stanie stwierdzić kto jest sprawcą zamachów. Nie wykluczono powiązania zamachowców z Al-Kaidą. "Prawdopodobieństwo udziału Al-Kaidy jest duże, ale za wcześnie, by konkretnie wskazać sprawcę" - powiedział pracownik londyńskiej policji.


'''Brytyjska Gorąca Linia Anty-terrorystyczna +44 0800789321'''