The move follows a meeting yesterday between Stephen Lander, the Security Service's director-general and Jack Straw, the Home Secretary.
David Shayler, who spent five years working for MI5, has caused a political furore after claiming that the Security Service kept personal files on dozens of prominent individuals including Mr Straw, Peter Mandelson, the minister without portfolio, Sir Edward Heath, the former Tory prime minister and Tam Dalyell, the Labour MP for Linlithgow.
Mr Straw and Mr Lander discussed the issues of accountability and surveillance at yesterday's meeting, which was described as "friendly and frank".
Detectives from the Metropolitan Police's Special Branch are to investigate Mr Shayler, who is in hiding in Europe, and will attempt to locate and interview him.
Mr Shayler could face up to two years' imprisonment if found guilty of breaking the Official Secrets Act.
The decision by MI5 to call on the police in an attempt to stop Mr Shayler making any more embarrassing and damaging disclosures was immediately challenged by Liberty, the civil-rights group, which has pledged to help the former agent fight any attempt to prosecute him.
Mr Shayler's revelations - which include allegations that the Security Service kept secret files on leading politicians, trade unionists and journalists - have raised questions about MI5's accountability, and, according to the former agent, show the organisation's obsession with gathering information on harmless individuals.
Under the Official Secrets Act 1989 it is an offence for any member of MI5 "to disclose without lawful authority any official information about security or intelligence".
Last night John Wadham, the director of Liberty, said that his organisation had contacted Mr Shayler and offered to help him fight any prosecution.
"It seems to us that the revelations are in the public interest and there should be no prosecution," Mr Wadham said.
"He [Mr Shayler] was acting in the best interests of our democratic society and for society to prosecute him would be perverse."
Mr Wadham added: "These examples demonstrate that MI5 hasn't been under control, under any democratic or legal controls. We have to raise questions about their existence."Reuse content