The UN declared any such operation would be illegal.

"This is something which is not entirely surprising because we always have suspected that," Andreas Nicklisch, deputy director of the UN's office in Brussels, told Reuters.

"It's illegal of course, but it's also unnecessary because we work in complete transparency and openness."

Short's claim comes a day after Blair's government dropped charges against a translator who admitted leaking a top-secret US document seeking London's help in bugging United Nations members in the run-up to the war.

Asked whether British spies had been told to carry out operations within the UN, Short replied: "Yes, absolutely."

The ex-aid minister, who resigned after the war but was in government during the period when London and Washington sought UN authorisation for military action, said Secretary General Annan's office had been specifically targeted.

"In the case of Kofi's office, it was being done for some time," she told BBC Radio. "I read some of the transcripts of the accounts of his conversations.

"In fact, I have had conversations with Kofi in the run-up to the war thinking 'oh dear, there will be a transcript of this and people will see what he and I are saying."'

A Blair spokesman refused to comment on the allegation beyond saying that the UK's security services "act in accordance with national and international law at all times."

But the prime minister himself will be pressed on the issue at his monthly press conference later in the day.

Iraq has become a political nightmare for Blair. Ten months after Saddam Hussein was toppled, none of the banned weapons he claimed Iraq had primed for use has been found.

The premier's public trust ratings have slumped and many in his Labour Party feel let down to the point of mutiny.

"Until the boil of the truth about Iraq is lanced, the prime minister can never put this behind him," Labour MP Jeremy Corbyn told Reuters.