Former Defence Secretary Tom King: ''This is an amazing dossier of information''

Mr King, chairman of the Parliamentary Security and Intelligence Committee, said the unmasking of 87-year-old Melita Norwood as a Soviet spy was just part of what has been described as "the most complete and extensive intelligence ever received from any source".

Her file was just one of many smuggled out Russia by a defecting KGB agent.

Mr King said it would be "incredible" if the home secretary had not been informed when Mrs Norwood's role became known to the British intelligence services.

He said his committee would be investigating the revelations as there now had to be a "full consideration" of the case.

KGB methods

And he said the files taken out of Russia by Vasili Mitrokhin, a former KGB officer, would reveal more names in the next few days.

Tom King: Committee will investigate

"We are just beginning a great period of exposure. There's a lot more to come," he told BBC Radio 4's Today programme.

"This is going to reveal a lot. I have seen a brief summary of some of the key areas and I think it's going to be of key interest not only to ourselves but also to the US."

He said Mrs Norwood's role may not have been made public earlier because there was much more information in the files to investigate.

Mr King said they covered the activities of the KGB "practically since the October revolution" of 1917.