A 65-year-old woman who works for the government has been arrested by counter-terrorism police on suspicion of an offence under the Official Secrets Act.
The Metropolitan Police said the contractor's arrest was made in north London on Wednesday morning and the woman is being held in a police station in the south of the city.
She was arrested on suspicion of an offence contrary to section 1 of the Official Secrets Act 1911, and was detained under the Police and Criminal Evidence Act.
A north London property is being searched in connection with the investigation which began after "intelligence was received", Scotland Yard said.
The government department which the woman works for has not been revealed.