The 23-year-old, who has not been named, has been detained under section 1 of the Official Secrets Act 1911 which covers espionage. It is understood he was on his way to pass on the information in Leeds when he was arrested.

The alleged security breach was from a government establishment and not from a commercial military supplier or manufacturer.

The man, who is understood to be British, lives in Skipton where a residential and business premises have been searched as part of an operation run by the Metropolitan Police. The man moved to the region relatively recently.

The information he is alleged to have tried to leak concerns the military and is believed to have been obtained during his previous occupation. He now has another unconnected job.

The alleged security breach does not relate to the surveillance bases at Menwith Hill, near Harrogate, or Fylingdales on the North York Moors.

The man was last night continuing to be questioned at a police station in Leeds after his arrest on Wednesday evening. He was also arrested under section 4 of the Explosive Substances Act 1883 after the discovery of certain material during the searches.

A spokesman at the Russian Embassy in London last night said he was unaware of the details surrounding the arrest.

The alleged informer is being represented by Leeds-based criminal lawyer Grahame Stowe.

He said: "My client is being held for questioning and we are awaiting further developments."

The Metropolitan Police issued a statement which said: "At approx 8.10pm on Wednesday officers arrested a man in his 20s at an address in Leeds under the Official Secrets Act 1911. The man was further arrested under section 4 of the Explosive Substances Act 1883.

"Following an application to court for an extension of custody the man remains in custody at a Yorkshire police station."

People working with sensitive information are commonly required to sign a statement to the effect that they agree to abide by the restrictions of the Official Secrets Act. The maximum sentence for being found guilty under section 1 of the 1911 Act is 14 years imprisonment.

News of the arrest comes only days after the director general of MI5, Jonathan Evans, warned of a continuing spy threat from Russia.

In a speech on Monday, Mr Evans said: "There has been no decrease in the numbers of Russian intelligence officers conducting covert activity in the UK, despite the Cold War ending nearly two decades ago. MI5 is expending resources to defend the UK against unreconstructed attempts by Russian, and others, to spy on the UK.

"The size and nature of this threat means that MI5 still has to devote significant amounts of equipment, money and staff to countering this threat, when they could be devoted to countering the threat from international terrorism – a threat to the whole international community and not just the UK."