The inquiry has been sparked by claims by Mr Tomlinson, who was dismissed by the service three years ago, that a national newspaper editor had been recruited as a paid agent for MI6.
He made the comment in an article published in the Sunday Business newspaper. As a result, their offices were visited last week by one of MI6's five directors and the editor was warned about breaching a High Court injunction.
This prevents newspapers from publishing any material relating to Mr Tomlinson's work with MI6 which is not in the public domain.
The Treasury Solicitor has also warned the Sunday Times newspaper about the injunction after it ran a story involving Mr Tomlinson which referred to an allegation that MI6 had a mole in the Bundesbank.
The story was orginally circulated on the Internet by two Swiss journalists after they obtained a copy of a letter which Mr Tomlinson had written to his solicitor in confidence about the matter.
Mr Tomlinson is currently living in Geneva, Switzerland, after serving six months in prison for attempting to write his memoirs about the time he spent with the security services.
John Wadham, Mr Tomlinson's solicitor, has said he does not believe that the Government would consider seeking his client's extradition after the failure to extradite the former MI5 officer David Shayler from France.Reuse content