The Special Immigration Appeals Commission (Siac) hearing the case ruled that suspicions about Katia Zatuliveter passing on secrets while working for a Liberal Democrat MP had been reasonably held but were wrong.
The ruling in her favour followed a lengthy hearing in which her relationship with Portsmouth South MP Mike Hancock came under intense scrutiny.
When Miss Zatuliveter was arrested in December 2010 as she was about to fly to Moscow, her former boss, Mr Hancock, immediately leapt to her defence and challenged the security services to prove their suspicions that she was a spy.
At the time, the 65-year-old politician was seen to be standing up for a young woman just starting out in her career.
But ever since it emerged that the 26-year-old had been having an affair with Mr Hancock, talk of a honey trap by the Russian has never been far away.
At the hearing in London to determine her deportation case, government lawyers argued that Ms Zatuliveter had exploited her position as Mr Hancock’s assistant and mistress to pass information to Russian intelligence.
They argued she should be deported because her presence endangered national security.
They claimed Ms Zatuliveter deliberately started having an affair with her boss, who sat on the Commons defence committee and was chairman of the all-party Russian group, because she saw him as “politically influential”.
Jonathan Glasson, lawyer for the home secretary, argued the MP was also known to have had previous extra-marital affairs and had been targeted because his private life made him “potentially vulnerable”.
I have managed to disable the work of half of Nato by distracting Y from his work. I can’t continue writing because the Kremlin are calling me to congratulate me on my achievements”
Katia Zatuliveter diary entry scrutinised in hearing
However throughout the hearing, Ms Zatuliveter and her lawyers maintained that the former parliamentary researcher had never been a Russian spy and her relationship with her former boss was genuine.
She told the hearing she had not thought Mr Hancock was influential as he was “a backbench MP”, and she had not known of his previous affairs before their own four-year relationship began.
The day after details of the couple’s affair was revealed, Mr Hancock resigned from his post on the defence select committee, saying the committee should be allowed to work unimpaired by the case.
As the hearing continued, government lawyers continued to build up an impression of a femme fatale, disclosing that Ms Zatuliveter had had a number of sexual relationships with officials from European countries, some when she had acted as a chaperone to delegates at conferences in Russia.
They had included a diplomat in his 30s when she was aged 18, and later, a high-ranking Nato official in his 50s, who she had begun seeing in April 2010, the commission heard.
But Ms Zatuliveter told the hearing she had worked at conferences while studying at St Petersburg University – which is where the British government believes she was signed up by Russian intelligence – so she could practise her language skills and learn “what is going on in the world and what people are talking about”.
Her lawyer, Tim Owen QC, argued his client’s diaries revealed her affairs were heartfelt, adding that a “joke” she had made about spying in one relationship was the “only reference” the secret services had from her about Nato, and that was now being used against her.
In a witness statement written by Ms Zatuliveter, in which she remembers an email conversation with around 50 to 100 emails, she said: “I recall writing something like ‘I have managed to disable the work of half of Nato by distracting Y from his work’.
“I can’t continue writing because the Kremlin are calling me to congratulate me on my achievements.”
Ms Zatuliveter’s diary during the period of her relationship with Mr Hancock also became a key focal point during the hearing, with Mr Owen arguing that the document – which the former parliamentary researcher submitted to MI5 – corroborated how she felt about the MP, who became her lover shortly after the pair had met in April 2006.
In one passage, she had written about worries that Mr Hancock had not called her. In another she wrote “at long last I will see my love after a long separation”.
Ms Zatuliveter was arrested in August 2010 at Gatwick airport as she was about to fly to Moscow
The security services said they believed her diary was possibly fake, written to lend her story credibility.
However when it emerged that MI5 had not tested the authenticity of the diary, it led the senior judge hearing the case, Mr Justice Mitting, to say it was “a weakness in the home secretary’s case”.
An MI5 officer conceded it was “regrettable” the diary had not been more thoroughly examined.
But the counsel for the Home Office said it maintained that the diaries did not prove Ms Zatuliveter was not a Russian agent.
The first counter-espionage appeal to come before the tribunal also heard other claims and counter-claims about Ms Zatuliveter’s case.
From government lawyers, it heard how the MP’s aide had had contact with a Russian, referred to as Boris, who worked at the country’s embassy in London and has since been expelled.
Ms Zatuliveter’s lawyers said there was “no actual evidence” of Miss Zatuliveter spying and accused MI5 of being more like bumbling Inspector Clouseau than fictional spy hero George Smiley.
Ms Zatuliveter’s lawyers also accused MI5 of showing interest in her only after Russian spy Anna Chapman had been deported from the US for spying in 2010, a suggestion which an MI5 agent known as “Witness ZZ” denied.
In its ruling, Siac concluded that MI5’s investigation had been “thorough and competent” and its suspicions were reasonably held, but ultimately proved wrong.
Of the honey trap suspicions, it added: “We are satisfied [Miss Zatuliveter] was not tasked to seduce.”