Greek police are to investigate allegations that a group of Pakistani men were illegally held and questioned about their UK connections.
Seven men who live in Athens say they were abducted in July shortly after the suicide bomb attacks in London.
They say their captors spoke English, blindfolded them and held them for at least 48 hours.
UK Foreign Secretary Jack Straw said allegations that British agents were involved were "complete nonsense".
He said no British agents had taken part in any of the alleged mistreatments.
In a statement, the British embassy spokeswoman in Athens said she would not comment on media speculation about alleged intelligence operations, no matter how far-fetched, as in this particular case.
"Media allegations on the detention of individuals in Greece are a matter for Greek authorities", the embassy statement added.
Greek police have also played down the allegations, pointing out that they only recently received a formal complaint about the alleged interrogations, despite the claims that the men were held and questioned in July.
The lawyer for the seven men, Frangiskos Ragoussis, told Greek radio that he believed his clients were questioned by British security officers.
"They were asked whether they had relatives in London, who they were in touch with, what those people's phone numbers were, whether they spoke to them before the bombings and where they lived," he said.
Mr Ragoussis said he hopes a petition now lodged with the Greek parliament will force the ministers of justice and public order to answer questions within the next few days.
He said he took his allegations to parliament after Greek prosecutors failed to act on an earlier complaint.
Public prosecutors have now written to police, triggering a standard investigation.
The Greek public order ministry said it had received no formal complaint on the alleged incident. "No Greek security services were involved," it said in a statement.
But Nikos Voutsis, of the small opposition Left Coalition, said his party had raised the issue four times with the government since July.
Javed Aslam, head of the Unity of Pakistan community association in Athens, filed a complaint on behalf of the men with an Athens prosecutors in July.
He told the Associated Press news agency that a total of 28 people were taken from their homes last July.
On 7 July, four presumed Islamic militant suicide bombers killed themselves and 52 others in four attacks on London underground trains and a bus.