"Worrying developments" in the last week led to the arrest of a Westminster parliamentary aide long suspected of spying for Russia.
The presence of 25-year-old Katia Zatuliveter in the office of Mike Hancock has been a source of concern to the security service for some time.
Security sources say there is no evidence of any wrongdoing on the part of the Liberal Democrat MP and confirm that no espionage charges will be brought against Ms Zatuliveter, but say her presence in this country is "not conductive to the national interest" and she will be deported.
In recent weeks, a number of questions about British nuclear and submarine capabilities have been submitted to the Government from Mr Hancock's office.
Although ministers have declined to answer some of the questions, Ministry of Defence and Foreign Office officials acknowledge that Mr Hancock, as a member of the Commons defence select committee, has cogent reasons to seek information on the nuclear issue, a matter of intense debate at a time of severe spending cuts with the Liberal Democrats opposed to the renewal of the Trident programme.
The MP for Portsmouth South, which has a large Royal Navy base, is also the vice-chairman of the all-party group on Russia, and last week it was reported that his colleagues at the Council of Europe, in Strasbourg, had been concerned enough about his voting record and alleged pro-Russian views to raise the matter with the then-Liberal Democrat leader, Charlie Kennedy.
There are also reports that Ms Zatuliveter had an affair with a Nato official who had access to classified material and this has caused further alarm. But a number of Alliance officials in Brussels said yesterday that they were unaware of any such relationship.
Mr Hancock defended Ms Zatuliveter yesterday, insisting that a mistake has been made and she should be freed.
Ms Zatuliveter, who has a Masters degree in peace studies from Bradford University, passed security vetting in order to join Mr Hancock's office in Parliament. Her family live in the south Caucasus where her father, Andrei, believed to be a former officer in the Russian army, has run a number of commercial concerns, including a gas trading company.
Ms Zatuliveter was reported to have been arrested by police and Border Agency officials and is being held at a secure facility awaiting deportation to Russia. She had been stopped and questioned last month at Gatwick airport after returning from a holiday in Croatia, with MI5 said to have referred extensively to her personal life.
When Vladimir Putin came to power in Russia, he significantly increased funding for the SVR (External Intelligence Service), the successor to his former employers, the KGB, and other Russian security services, and there have been persistent reports from the security agencies in the UK about the scale of Russian espionage activities, although it remains unclear just how successful they have been.
Ms Zatuliveter's arrest comes five months after a "Russian spy ring" was uncovered in the US with a young woman, known as Anna Chapman, supposedly playing a key role.
However, despite a deluge of publicity – especially about Ms Chapman, the "Mata Hari" who, according to one American tabloid, had "Sexy Red Agent's Locks to Die For" – the members of the "spy ring" did not face any spying-related charges, and were accused, instead, of failing to register with the US authorities as representatives of a foreign power and omitting to declare income for tax purposes.
Peter B Krupp, lawyer for one of those arrested, said after studying FBI affidavits: "The government's case essentially suggests they successfully infiltrated neighbourhoods, cocktail parties and the Parents and Teachers Association." The "agents" were, eventually, not tried but deported to Russia.
Andrei Zatuliveter was asked at his home in Zmeika by the representative of a British newspaper whether his daughter, the "Sexy Russian Blonde" of the headlines, had any connection with Ms Chapman.
He allegedly screamed: "Get out, I'll kill you, bastard", before chasing the journalist through the village, first on foot and then in his car. Later, in a telephone conversation, he said: "I don't believe any of this rubbish."
Mr Hancock also reiterated that he believed the allegations against his aide were rubbish. He stated: "No one has ever said to me under any circumstances whatsoever that she has been involved in anything like that. She was already in this country so she didn't need a visa. She was educated here – she did a Masters – and that gave her the right to work for a period and then it was renewed. The whole thing has been blown out of all proportion."
The MP added: "It's not my problem. It is now in the hands of her lawyers. I am sure that in the end she will be proved to be right."
The shadow Foreign Secretary Yvette Cooper said vetting procedures for those applying to work in the Commons might have to be looked at. "It is important to make sure those are strong enough and secure enough.
"I'm sure that depending on what happens with this individual case, if there do turn out to have been breaches in security, then obviously the wider security in Parliament would be looked at."