MI5 used hidden electronic surveillance equipment to secretly monitor 10 Downing Street, the Cabinet and at least five Prime Ministers, The Mail on Sunday can reveal.
The extraordinary disclosure comes despite a succession of parliamentary statements that no such bugging ever took place.
And it follows a behind-the-scenes row in which senior Whitehall civil servants – backed by Prime Minister Gordon Brown – attempted to suppress the revelation.
The Mail on Sunday has learned that top-secret files held by the Security Service show it installed electronic listening devices in three highly sensitive areas of No10 – the Cabinet Room, the Waiting Room and the Prime Minister’s study.
It means that for nearly 15 years, all Cabinet meetings, the offices of senior officials and all visitors to the Prime Minister – including foreign leaders – were being bugged.
The disclosure is highly shocking in its own right but it will also bring genuine concerns as to why the Cabinet Office still wants to suppress it.
Comments from MI5 chief Jonathan Evans suggest that the attempted block was not done on grounds of national security but for wider public interest reasons.