A woman has been arrested by counter-terrorism police on suspicion of breaching the Official Secrets Act, the Metropolitan Police has said.
The 65-year-old had been working for the government and was detained after intelligence was received by police.
Scotland Yard said she was arrested at a property in north London under the Police and Criminal Evidence Act.
The woman is in custody at a south London police station and a north London address is being searched.
Police have not identified the government department she was working for as a contractor.
BBC home affairs correspondent Daniel Sandford said the fact the woman is being held under Section 1 of the Official Secrets Act 1911 means she is suspected of spying.