Russia's foreign ministry has imposed tight travel restrictions on British diplomats working in the country.
It said they must give at least five days' notice and provide extensive travel details if they want to leave a 120km (75-mile) "free movement zone".
Only the British ambassador and three senior diplomats are exempted from the measures, the ministry said.
It comes amid worsening relations between Moscow and London following the invasion of Ukraine.
Despite the two countries being at odds over the war, both have continued to operate diplomatic missions on each other's soil.
The Vienna Convention - which the UK and Russia are both signed up to - is clear that governments must give accredited diplomats "freedom of movement and travel in its territory".
So governments rarely actually stop diplomats from travelling - but they can make it very difficult by imposing rules and regulations, such as the Russian government has.
The Russian foreign ministry said British diplomats will have to provide details of accommodation, transport, planned contacts and the purpose of any trip before travelling within Russia.
The restrictions will also be applied to diplomats working at the British consulate in Yekaterinburg, a city around 1090km (880 miles) to the east of Moscow.
The resulting bureaucratic demands are time-consuming and onerous but are just within the letter of the Vienna Convention.
In truth, this is just one of many irregular ways of making life difficult for diplomats.
They could find visa applications take time. They and their families could be subject to greater or lesser surveillance.
One diplomat once told me that she returned to her flat in Moscow one evening to find the magnetised letters on her fridge had been re-arranged to spell FSB, the Russia security service.
Another found her cat frozen to death outside her flat. She suspected it had been shut out deliberately.
So it is not unusual for authoritarian states to make life difficult for diplomats. For many, it goes with the territory.
The announcement was made shortly after the UK's interim charge d'affaires attended a meeting with Russian officials and was informed of the decision.
The UK Foreign Office has disputed the Russian foreign ministry's claim that the senior British diplomat had been "summoned", describing it as "disinformation".
A spokesperson for the department said: "This was a planned meeting, held at our request, as part of standard diplomatic practice."
In a statement confirming the restrictions, Russia cited the UK's support of the Ukrainian government.
It accused the UK of conducting "hostile actions... including the obstruction of the normal functioning of Russian diplomatic offices in the UK".
The UK Foreign Office has not said how it will respond to the move.