British spies would have undermined the United Nations' work if it was true they bugged Secretary General Kofi Annan's office, says a UN spokesman.
Ms Short says she expects there are transcripts of calls to Mr Annan
Fred Eckhard told reporters Mr Annan wanted the action stopped if it was happening, especially as it would have been illegal.
He spoke out after ex-cabinet minister Clare Short's claim that the UK had been listening in to Mr Annan's talks.
Tony Blair said Ms Short's claims were "deeply irresponsible".
Mr Blair also appeared to cast doubt on her future as a Labour MP.
Mr Eckhard said Mr Annan had spoken to the British ambassador to the UN about the bugging claims.
"We want this action to stop if indeed it has been carried
out," he said.
"It undermines the secretary-general's conduct of business with other leaders. It is therefore not good for the United Nations' work and it is illegal."
Clare Short has been a thorn in the government's side since she quit
The UN would be disappointed if the accusations proved to be true, he added.
At his monthly news conference, the prime minister insisted the UK security services acted in accordance with domestic and international law and in the best interests of this country.
Mr Blair said: "I'm not going to comment on the work of our security services - do not take that as an indication that the allegations made by Clare Short are true.
"I really do regard what Clare Short has said this morning as totally irresponsible, and entirely consistent."
Asked whether she should be prosecuted or face Labour Party discipline Mr Blair said he would "have to reflect upon" her comments.
He added: "There will obviously be issues that arise... I am not in a position to answer them at the moment."
Ms Short's comments came the day after the dramatic collapse of the trial of GCHQ whistle-blower Katharine Gun.
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.
But it has denied claims the move to drop the prosecution was politically motivated.
There has been speculation ministers were worried about the disclosure of secret documents during the trial, particularly the advice from Attorney General Lord Goldsmith about the legality of war.
Lord Goldsmith said in a statement to the House of Lords: "It was a decision on solely legal grounds... and free from any political interference."
Spies there 'for some time'
He said that although they believed they could prove the Official Secrets Act had been breached, they had concluded they could not disprove Mrs Gun's defence "of necessity" - believed to refer to her case that she felt a duty to do something to save lives in an unlawful war.
However, Mr Blair said it would be a "very dangerous situation" if people thought they could just "spill out allegations, whether false or true... and get away with it".
In an interview with Newsnight to be broadcast at 2230 GMT, Ms Short 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.
Earlier, she told BBC Radio 4's Today programme that British spies were involved in bugging Mr Annan's office in the run up to war with Iraq.
"The UK in this time was also getting spies on Kofi Annan's office and getting reports from him about what was going on," she said.
"These things are done and in the case of Kofi's office, it was being done for some time."
Asked if Britain was involved in this, she replied; "Well I know - I've seen transcripts of Kofi Annan's conversations.
BBC security correspondent Frank Gardner said that many UN officials always worked on the basis that they were being bugged.
But, he added, "that is not to say that it is acceptable if they are not suspected of terrorism or other crimes".
Conservative leader Michael Howard said the situation was "a complete mess".
"It's about time the prime minister got a grip on it and sorted it out," he said.
Liberal Democrat leader Charles Kennedy said it was not good enough for Mr Blair to say he could not comment on the security services - especially after publishing intelligence in his Iraq dossiers.
"Tony Blair must now come clean about this central accusation," he said.