Downing Street has expressed its "surprise" at Clare Short's broadside against the prime minister.

Number 10 would not confirm whether the international development secretary has the full support of the prime minister.

Tony Blair's spokesman refused to say whether she would remain in government following her blistering attack.

The spokesman would not deny claims from the lobby that the prime minister had cut the minister adrift.

Number 10 said the Labour leader held a one-to-one meeting with Short on Friday. At no point had she raised her concerns, the spokesman claimed.

Blair spoke "briefly" to Short last night and again this morning.

The prime minister, who has cleared his diary to engage in round the clock diplomacy, is said to be furious at Short's attack.

"He is not going to be diverted onto other issues at this stage," said Number 10.

Short, international development secretary since 1997, accused Blair of recklessness on Iraq and warned she would quit unless the UK secures a second United Nations resolution.

Warning that it is time to put her "cards on the table", she confirmed that she would leave government if Blair goes to war without the backing of the UN.

"If there is not UN authority for military action or if there is not UN authority for the reconstruction of the country, I will not uphold a breach of international law or this undermining of the UN and I will resign from the government," she told the BBC last night.

"I think it's time for cards on the table. People are speculating and making all sorts of statements about my intentions. I think I owe it to my colleagues in the government and members of the Labour Party to just be truthful about my position. It's the time to say what my intentions are."

Short said the prime minister's actions in relation to Iraq had been "deeply reckless".

Shadow foreign secretary Michael Ancram said today that Short should resign after she had put "the credibility of the Blair government" at stake.

"It is an astonishing and damaging departure from the whole principle of collective responsibility that the prime minister can retain in his Cabinet a member who has openly described him and his policy as 'reckless'", he said.

"His [Blair's] credibility and his ability to undertake the delicate negotiations and vital decisions which lie ahead will be fatally damaged by the continued presence of Clare Short within the Cabinet."

Short's intervention is a severe blow to the prime minister who on Sunday faced his first government resignation over the looming Iraq war.

Andy Reed, the parliamentary private secretary (PPS) to Margaret Beckett, announced his decision to quit amid increased speculation that Britain will not secure a second UN resolution.

"I will set out my full reasons for resigning, but I fully support the prime minister in his attempts to find a peaceful solution to the Iraq crisis through the UN route and do not want to do anything that undermines that effort at this stage," said Reed on his website.

"I still oppose any war without specific UN agreement along with many other Labour colleagues."

Deputy prime minister John Prescott admitted there was considerable Labour unease on the issue.

"This is a very serious issue. Lots of MPs are concerned about it. Lots of the public are concerned about it," he told Breakfast with Frost.

Reports suggest that up to 10 ministerial bag carriers may be set to resign in protest at the government's Iraq policy.

Ken Purchase, an aide to Commons leader Robin Cook, has signalled that he will quit if the prime minister goes ahead with his war plans.

Anne Campbell, PPS to trade secretary Patricia Hewitt, has also warned that she will resign unless a second resolution is achieved.

Said Hewitt: "She has told me that she would be very unhappy about the possibility of war without a second resolution.

"I have to say I think it is a bit self-indulgent really for people to be talking about resignation in the hypothetical situation that there is no second resolution when the government is working flat out to get that second resolution."

Tony Wright, PPS to the attorney general Lord Goldsmith, said the UN "is the only route" available to the government.

"If there is a veto then that would put me - and I suspect a considerable number of others - in a very difficult position. Yes, I would quit," he said this weekend.

Former minister Tony Banks said a war would have "pretty devastating" consequences for the government.

Scores of Labour MPs who backed the prime minister in the recent Iraq vote have also signalled that they are set to join the rebels.

It emerged yesterday that at least one Constituency Labour Party (CLP) will demand Blair's resignation if he goes to war without UN support.

The motion, tabled by Linlithgow CLP, states: "We invite the prime minister to consider his position as Leader of the Labour Party if he goes ahead with war."

Several pro-war Labour MPs are also being warned that they may be deselected by their local parties unless they reconsider their stance.

Lib Dem foreign affairs spokesman Menzies Campbell said the prime minister's job was now on the line.

"Without a second resolution, Mr Blair will not only lack Clare Short's support but also that of the majority of people in the UK," he said.

"By allying himself so closely with the President, the Prime Minister has put his fate in the hands of George W Bush."