An MI6 operative who tried to sell state secrets to a foreign Government for £2 million has been jailed for 12 months.

Daniel Houghton, 25, will be released almost immediately because of time served in custody and plans to return to live in his childhood home on Dartmoor after he is released.

Intelligence experts fear top-secret files he stole to sell to Dutch intelligence agents could still fall into enemy hands. Many documents containing details of secret information-gathering software he devised have yet to be traced.

Houghton, a software engineer living in Hoxton, east London, was jailed yesterday after pleading guilty to two offences under the Official Secrets Act at an earlier hearing.

Mr Justice Bean told him: "The effect on the SIS (Secret Intelligence Service) credibility and the morale of its officers of this kind of act of betrayal is a serious matter."

The judge said he did not know whether it was true, as Houghton claimed, that he was hearing voices that told him to do it but said he was a "strange young man".

Houghton, a former Devon schoolboy, was caught after offering to hand over sensitive computer files containing information about intelligence collection and MI6 staff lists to agents from the Netherlands.

The Dutch at first thought it was a hoax but later tipped off their UK counterparts and Houghton was arrested after arranging a meeting at a London hotel in March this year.

The court heard that Houghton tried to sell two secret staff lists, one containing the names of 387 individuals and the other with the home and mobile telephone numbers of 39 individuals.

Piers Arnold, prosecuting, said: "It was a personal betrayal of these individuals with the potential if it had fallen into the wrong hands to compromise individuals' safety."

Houghton had worked for the SIS, also known as MI6, between September 2007 and May 2009, the court heard.

During this time he accessed "a number of computer files" belonging to the British Security Service (MI5) relating to the work of both intelligence agencies – marked "secret" or "top secret".

The 25-year-old boasted he made copies of the electronic files as he attempted to sell them. As he negotiated for cash, Houghton revealed he had a second memory card containing further information hidden at Littlecombe Farm, at Holne, near Buckfastleigh, where his mother Elizabeth Havinga lives. This has never been found.

Investigators feared Houghton, who holds joint British and Dutch nationality and speaks Dutch, could use his £32,000 savings to flee the country as he had few social ties.

Houghton, who claimed he was "directed by voices", showed astonishing naivety for someone selling highly sensitive state secrets.

When he first made contact with the Dutch agents, he used his own mobile phone to call a publicly-listed number.

One source close to the inquiry said he was a computer expert with limited social skills.

He said: "He certainly was not James Bond."

He added that the move to sell the information was not sophisticated and investigators had been unable to find any other attempts.

David Perry QC, defending, said: "This was not an offence committed by a calculating ideologue to disclose material to a hostile sovereign state."

He added that the offences were "incompetently executed" and it was clear that Houghton would be caught.

Mr Perry said the defendant, who came from a "supportive" family, had no previous convictions and would go to live with his mother in Devon on his release.

Mr Justice Bean told Houghton that selling staff lists "would have disclosed the identity and homes and whereabouts of agents whose identity must be protected almost at all costs".

"If the material had found its way into the hands of a hostile power it would have done enormous damage and put lives at risk," he added. "On the other hand, you are not an ideologue. If you had been intent on causing harm to this country's interests you would have chosen a different recipient than the Netherlands."

The judge said he had been unable to resolve conflicting psychiatric reports, and because he had already served 184 days he could be released "as soon as the formalities can be completed".