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An alleged Russian spy who had an affair with Liberal Democrat MP Mike Hancock and worked for him has won her appeal against deportation.

The Special Immigration Appeals Commission (Siac) said it had seen nothing to prove Katia Zatuliveter was a spy tasked to seduce Mr Hancock.

The 26-year-old worked as a parliamentary researcher for the 65-year-old Portsmouth South MP.

The �鶹Լ�� Office said it was disappointed but stood by its decision.

Mr Justice Mitting, the president of Siac, said: "Nothing in the material which we have analysed suggests, let alone demonstrates, that the appellant exploited her relationships for the purposes of the Russian state."

The panel, which included the former MI5 chief Sir Stephen Lander, said that her relationship with the MP had been "enduring and genuine".

The appeal, most of which was held in secret, heard that MI5 assessed that Miss Zatuliveter had been placed in Mr Hancock's office by Russian agents because he was a member of the Commons defence select committee.

Mr Hancock, a backbench MP, said he only saw defence documents which were in the public domain and denied passing any secrets on to the Russian, with whom he had a four-year affair.

Siac ruled Miss Zatuliveter had not been liaising with Russian spies from either the FSB or SVR agencies and that even if she had, "we have seen nothing which satisfies us that she was recruited as an agent or was tasked, or acted, as one".

Miss Zatuliveter left Tuesday's hearing smiling and clutching a bouquet of flowers from wellwishers. Her solicitor, Tessa Gregory, denounced the "amateur" nature of the case against her client.

She said: "Today is a historic judgement. Siac has exonerated Katia. The court did not reach that conclusion by a narrow margin.

"The security service's case was found to be wanting at every stage. Many of the factors relied upon by the security service led the court to an opposing conclusion.

"Their case was built entirely on speculation, prejudice and conjecture. It was amateur. We hope the government will reflect very carefully on today's judgement which must raise serious concerns about the standards of professionalism and competency within the security service."

A �鶹Լ�� Office spokesman said: "National security is the primary duty of government and we will take all necessary steps to protect the public from individuals we believe pose a threat and remove them from the UK.

"The court ruled that there were ample grounds for suspicion. We are therefore very disappointed by the court's judgement and stand by our decision to pursue deportation on national security grounds."

Miss Zatuliveter told the semi-secret tribunal that she had met Mr Hancock in St Petersburg in 2006 and fell in love with him.

She denied their relationship was based on sex and told the tribunal: "I don't know how people usually have relationships but when I have a relationship, it's based on communication."

Her defence team said her comprehensive diary proved the relationship had been genuine and criticised the security service for suggesting the diary might be fake, without conducting scientific tests to prove its age.

Mr Justice Mitting said the diary's account of the affair had a "ring of truth", adding: "The most likely explanation, and one which we find to be proved on the balance of probabilities, is that, however odd it might seem, she fell for him.

"Her activities would have been of great interest to the FSB/SVR, but they are also entirely consistent with her being an ambitious young woman with an intense interest in politics and international relations.

"Further, they would have been of no use to the FSB/SVR unless their product was communicated to them, of which there is no open evidence."

Siac dismissed criticisms of MI5, which was described by Miss Zatuliveter's counsel Tim Owen QC as acting "more like Inspector Clouseau than George Smiley".

Mr Justice Mitting said the criticism was "unjustified" and that the investigation had been thorough and competent. There had been ample grounds for suspicions which had been reasonably held, he said.

Miss Zatuliveter has a visa to stay in the UK until August 2012.