© The TelegraphDaniel Houghton
MI6 face questions over their security procedures after a computer expert was allowed to smuggle top secret files out of their headquarters which he planned to sell to a foreign power for £2m.

Daniel Houghton, 25, was caught by MI5 in a sting operation in a central London hotel as he tried to leave with £900,000 cash in a suitcase.

Under cover officers had set him up after a tip off from a foreign intelligence agency which said he had offered to sell them information.

Houghton, a computer programmer by training, had helped develop a cutting edge technique for intercepting emails, sources told the Daily Telegraph.

In order to drive up the price of the material he was selling, he also stole staff lists and home and mobile telephone numbers of MI5 and MI6 officers.

The information, which Houghton smuggled out of MI6's Vauxhall Cross headquarters, was labeled "top secret" and "secret."

If leaked, it would have had a "severe impact on operational capabilities and particularly the ability to collect intelligence," one security source said.

"He knew he had a valuable secret and wanted to make money out of it. His motivation was essentially greed."

MI6, the Secret Intelligence Service, employ IT security measures which stop users copying files and conduct bag searches but their most important line of defence is the vetting of their staff.

"Nothing is 100 per cent," another source said. "You have got to rely on everything coming together. We are happy that this is difficult to do and he didn't get very far, thankfully."

Yesterday Houghton pleaded guilty to two charges under the Official Secrets Act and was told that he "inevitably" faced a custodial sentence.

Piers Arnold, prosecuting, told a hearing in London, that Houghton claimed he was "directed by voices" to try and sell the secrets and the judge adjourned the case for further psychiatric reports.

MI6 knew nothing of what Houghton was doing until he contacted the Dutch intelligence service, the AIVD in August last year.

Houghton, who has dual British and Dutch nationality, worked for MI6 between September 2007 and May 2009, after leaving Birmingham University where he had studied interactive computer systems.

He downloaded information onto a number of CDs and DVD disks which he then copied onto a secure digital memory card.

Police later found a Sony memory card containing around 7,000 files, including deleted files, and a portable hard drive, under the bed at his shared flat in Hoxton, East London. It included nine MI5 operation files relating to the intercept project.

They also found a list of over 300 named MI5 and MI6 officers and home and mobile telephone numbers of 39 officers.

Sources said material was marked "top secret," "secret" and "restricted."

It took Houghton several months to persuade the Dutch that he was serious about selling secrets and then MI5 were able to set up a sting operation in February this year.

Undercover British and Dutch officers arranged to meet him on February 8 to view the material on his laptop while recording the meeting with hidden listening devices.

Houghton initially demanded £2m for the files but eventually agreed a price of £900,000 and they arranged to meet him again at a central London hotel on March 1 where he showed them the material on a laptop and then handed over two memory cards and a computer hard drive. He was arrested by police as he made his way to the hotel lift.

Yesterday Houghton appeared in court where he pleaded guilty to two charges under the Official Secrets Act concerning the unlawful disclosure of material relating to security or intelligence. He denied one count of theft.

Duncan Penny, defending, said: "He accepts that a custodial sentence is inevitable in this case, given the gravity of the offences concerned" but said it was not suggested there should be "a psychiatric disposal" of the case when it came to sentencing.