British counter-terorism police said they had arrested a woman on Wednesday under the Official Secrets Act.
Although the arrest was carried out by Scotland Yard's counter-terrorism officers the investigation, according to Whitehall sources, relates to alleged supplying of classified information to another state or organisation rather than terrorism.
The woman is in custody at a south London police station and a north London address is being searched.
The 65-year-old had been working for the government and was detained after intelligence was received by police.
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A Government spokeswoman said: "An individual who was contracted to work for the Government was arrested today for an offence contrary to section 1 of the Official Secrets Act, 1911".
It states that "i$3 f any person for any objective prejudicial to the safety or interests of the State." including making "any sketch, plan, model, or note which is calculated to be or might be or is meant to be directly or indirectly useful to an enemy".
A United Kingdom government contractor has been arrested by counter-terrorism police on suspicion of disclosing security or intelligence materials.
"It would not be appropriate to comment further as there is an ongoing police investigation".
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