Former cabinet minister Clare Short has denied putting the UK or its security services at risk by claiming UN chief Kofi Annan's phone calls were bugged.
Ms Short says she expects there are transcripts of calls to Mr Annan
After Tony Blair branded the claims "deeply irresponsible" Ms Short hit back by accusing the prime minister of using "pompous" distraction tactics.
She told BBC Two's Newsnight there was no national interest that justified spying on the UN secretary general.
The UN has said if the claims were true the UK had undermined Mr Annan's work.
Chief spokesman Fred Eckhard said any bugging would be illegal and should be stopped.
At his monthly news conference, the prime minister insisted the UK security services acted in accordance with domestic and international law and in their country's best interests.
Mr Blair did not directly deny the bugging operation took place, but did say Ms Short had been "totally irresponsible" and hinted she may be censured.
Other Labour figures have also rallied against Ms Short.
Her former deputy, George Foulkes, said she had no evidence for her claims and the fisheries minister, Ben Bradshaw, accused her of impugning Mr Blair's integrity.
In Friday's Independent newspaper, ex-Foreign Secretary Robin Cook said he would be "surprised" if Ms Short's claims were true.
A former British ambassador to the UN, Sir Crispin Tickell, told BBC Radio 4's Today programme that Ms Short had committed a "major breach of confidence".
But he said bugging was not necessarily "a bad thing" if done in the national interest - and he doubted whether the claim affected diplomatic trust.
In her Newsnight interview Ms Short was defiant and dismissed the prime minister's attack on her as a distraction.
"What's he going to say? He either says 'yes, it's true' or he has to say 'no, it's not true', then he would be telling a lie. So he's got to say something else, so he can have a go at me."
She said it was "pompous" of Mr Blair to claim she had threatened national security or the security services.
"There is no national interest here, there is absolutely no threat to the security services from me making this public," she said.
Ms Short says she saw transcripts of Mr Annan's phone conversations.
Asked if it was possible the UN had taped the conversations itself, she replied: "I don't think this matters. Someone is improperly distributing transcripts."
The former minister also said she had seen no evidence of spying operations against other UN diplomatic missions.
The British ambassador to the UN phoned Mr Annan about the claims on Thursday.
Kofi Annan has spoken to Britain's ambassador to the UN about the claims
But UN communications director, Edward Mortimer, said he did not think Mr Annan had received assurances the bugging had not happened or would not happen again.
Ms Short's comments came the day after the collapse of the trial of Katharine Gun, a whistle-blower at the government surveillance and communications organisation GCHQ.
She had been accused of leaking a secret e-mail from US spies apparently requesting British help in bugging UN delegates ahead of the Iraq invasion.
The government says it will review whether changes are needed to the Official Secrets Act in the wake of the case.
Conservative leader Michael Howard said the situation was "a complete mess" while
Liberal Democrat Charles Kennedy urged Mr Blair to "come clean".