Britain spied on U.N. Secretary-General Kofi Annan (search) in the build up to the Iraq war, a former Cabinet minister said Thursday, triggering yet another postwar crisis for Prime Minister Tony Blair.

Blair refused to confirm or deny the accusation and branded his former international development secretary, Clare Short (search), "deeply irresponsible" for commenting on sensitive security issues.

For Blair, the allegation is another potentially damaging aftershock of the Iraq invasion, following controversies over Britain's prewar intelligence dossiers, the death of a weapons scientist, the coalition's failure to find weapons of mass destruction and the collapse of a court case on alleged U.S.-British bugging of the United Nations.

But in a poised performance at his monthly news conference, the prime minister insisted British spies always acted within international law.

The United Nations said any spying on Annan's office would be illegal.

"Such activities would undermine the integrity and confidential nature of diplomatic exchanges. Those who speak to the secretary-general are entitled to assume that their exchanges are confidential," U.N. spokesman Fred Eckhard said.

In Washington, Secretary of State Colin Powell had no comment on the case. "I have nothing to say with respect to the activities in the United Kingdom. We never talk about intelligence matters of that nature. "

The opposition was quick to criticize Blair's government.

"I'm afraid the situation now seems to be a complete mess. It's about time the prime minister got a grip on it and sorted it out," said Michael Howard, leader of the main opposition Conservative Party.

Charles Kennedy, leader of the Liberal Democrats, said Blair "must come clean" and "reassure the British people that his government was not involved in spying on Kofi Annan."

Short, who has repeatedly embarrassed Blair since she quit the Cabinet in May over the war, said she read transcripts of Annan's conversations while she was a member of the government.

"The U.K. in this time was also getting, spying on Kofi Annan's office and getting reports from him about what was going on," she said in an interview with British Broadcasting Corp. radio.

"These things are done. And in the case of Kofi's office, it's been done for some time," added Short, who has accused Blair of being "reckless" and misleading the country, and has repeatedly called on him to resign.

Asked explicitly whether British spies had been instructed to carry out operations within the United Nations on people such as Annan, she said: "Yes, absolutely."

She made no comment on the method of spying on Annan.

Blair refused to comment directly on the allegation, and stressed his silence was not an indication it was true.

"I'm not going to comment on the operations of our security services," he said. "But I do say this: we act in accordance with domestic and international law, and we act in the best interests of this country, and our security services are a vital part of the protection of this country."

It is not the first time Britain has been accused of spying on foreign diplomats.

In December, Pakistan asked Blair's government to respond to a newspaper report that British intelligence agents had attempted to plant listening devices at its embassy in London. President Pervez Musharraf said Britain's failure to respond strained relations between the two countries.

The European Union revealed last March that bugging devices planted by an undisclosed country were found on phone lines of several nations -- including Britain -- in the building used for EU summits.

Short's allegation came a day after the collapse of a criminal case against a British intelligence agency worker who admitted leaking a document disclosing a U.S. appeal for British help in monitoring phones and e-mail traffic of members of the U.N. Security Council, when the two countries were seeking the council's backing for war.

Opposition politicians have questioned whether the government intervened to drop the case against Katherine Gun (search), fearing a trial would probe the legal argument for going to war.

Gun, 29, a former Mandarin translator with Britain's Government Communications Headquarters listening station, leaked a Jan. 31, 2003 memo from U.S. intelligence officers asking their British counterparts to spy on members of the U.N. Security Council in advance of a key vote on Iraq.

In preparing her defense, Gun's lawyers demanded the government disclose the advice it received from Attorney General Lord Goldsmith on the legality of going to war.

Goldsmith told the House of Lords the decision to drop the case was made solely on legal grounds and "free from any political interference."

The government said Thursday it would review the working of the Official Secrets Act, under which Gun was charged, saying it was disappointed with the collapse of the case.

Blair's decision to back President Bush caused his popularity to sag. Despite the swift fall of Baghdad, his personal ratings show little signs of recovering.

British intelligence dossiers claiming Iraq had an active and growing program of weapons of mass destruction -- Britain's principal rationale for joining the conflict -- have not been validated by evidence on the ground.

The government's bitter row with the BBC, over a report that it "sexed up" the threat posed by Iraq, ensured months of negative headlines and the worst crisis of Blair's career when a weapons scientist at the center of the allegations committed suicide.