A 65-year-old woman suspected of passing state secrets to countries such as Russia or China was arrested by anti-terrorism officers yesterday.

The woman, a British national who has been working for as a contractor for a Whitehall department, was held at her north London home by officers acting on a tip-off from the intelligence services.

She was arrested by Scotland Yard's counter-terrorism command on suspicion of an offence under the Official Secrets Act, which covers espionage and passing secrets to an enemy.

Officers yesterday searched an address in north London as part of the investigation.

Sources said the arrest was not related to either Islamist terrorism or Irish republican dissidents, and there is speculation the woman had been passing secrets to one of a number of foreign Governments, including Russia, China or even North Korea.

Section 1 of the act is headed 'penalties for spying' and covers activities that amount to an offence carried out 'for any purpose prejudicial to the safety or interests of the State'.

Scotland Yard did not identified the government department for which the woman worked.

She is not thought to have been working for any of the intelligence agencies, although the arrest was understood to have come as a result of information from MI5.