Russia's state security service, the FSB, has accused British diplomats of spying in Moscow
It backed claims made in a Russian TV report which showed footage of what it said was British agents retrieving data from a fake rock planted on a street.
The programme also said a UK diplomat made regular payments to Russian non-governmental organisations.
The TV report showed what it said was a UK diplomatic spy
The UK Foreign Office said it was "concerned and surprised", and denied any improper conduct with Russian NGOs.
Reuters news agency quoted an unnamed FSB spokesman as saying that "everything that was shown [in the programme] was true and based on our information".
The programme said four officials from the UK embassy and one Russian citizen, allegedly recruited by the British secret service, downloaded classified data from a transmitter in the rock onto palm-top computers.
According to the programme, the Russian citizen was later arrested.
Hidden camera footage appears to show individuals walking up to the rock.
One man is caught on camera carrying it away.
The programme contained a number of interviews with people claiming to be Russian intelligence officers, who made the allegations.
An unnamed FSB spokesman told AFP news agency one of the accused diplomats was a 30-year-old archivist.
A FSB officer told Rossiya television the hi-tech stone was "absolutely new spy technology".
The UK embassy in Moscow has refused to comment, but the UK Foreign Office in London issued a statement.
"We are concerned and surprised at these allegations. We reject any allegation of improper conduct in our dealing with Russian NGOs," it said.
The Foreign Office said it was well known that the UK government had given financial support to projects implemented by Russian NGOs in the field of human rights and civil society.
Russian TV also showed what it said was the spy rock
"All our assistance is given openly and aims to support the development of a healthy civil society in Russia," the statement said.
Russian President Vladimir Putin drew widespread criticism earlier this month when he signed a law giving authorities increased powers to monitor the activities and finances of NGOs.
Critics said the measure was an attack on human rights and democracy.
Michael Evans, defence editor of the UK Times newspaper, told the BBC that Russia is still regarded as a centre of espionage.
"People will be a little bit surprised at the bizarre nature of this episode. I've no idea whether it's true, but clearly there is a lot of intelligence gathering that goes on."