Tony Blair is tonight facing the possibility of mutiny within Labour ranks as Britain prepares for war against Iraq.

Following Robin Cook's resignation, Clare Short is now said to be considering her position.

The resignations come ahead of tomorrow's critical debate on the war with Iraq, before which the prime minister will address members of the parliamentary Labour Party.

The foreign secretary said on Monday that the Commons debate would be "the most important in many years".

A motion against military action has been amended to include support for troops stationed in the Gulf.

It states that "the case for war against Iraq has not yet been established, especially given the absence of specific United Nations authorisation".

The motion adds that "in the event that hostilities do commence" the Commons should pledge "its total support for the British forces engaged in the Middle East".

Signatories include former culture secretary Chris Smith, former defence minister Peter Kilfoyle and Liberal Democrat foreign affairs spokesman Menzies Campbell.

Critics of the government's policy predict that the number of rebels could exceed the 122 who defied a three-line whip in a recent vote on the crisis.

Cook announced his resignation from the government after the collapse of efforts to find a diplomatic solution to the Iraq crisis.

In a brief statement, the leader of the Commons and former foreign secretary said he could not back war.

"It is with regret I have today resigned from its Cabinet. I can't accept collective responsibility for the decision to commit Britain now to military action in Iraq without international agreement or domestic support," he said.

Cook left the government as a Cabinet meeting got underway in Downing Street.

Environment secretary Margaret Beckett said she "genuinely did not know" why Cook felt the need to resign.

"I have seen his statement. Of course I respect Robin, but I am afraid I profoundly disagree with him," she told Channel 4 News.

"I heard what he said last week in Cabinet. Of course he has concerns and anxieties.

"Everybody around the table has concerns. Everybody has anxieties, including the prime minister. But none of the rest of us feel that they are enough to make us judge that the government is doing the wrong thing."

"We believe in the end - hard though it is, difficult though it is, much though nobody wants conflict - the Government is where, unfortunately, we have a duty to be," she added, dismissing suggestions that the former Commons leader had been forced to quit the cabinet.

Blair's decision to support the US in a war against Iraq without United Nations backing has triggered a crisis within the Labour Party.

The prime minister will lead a debate in the Commons tomorrow. MPs will have a chance to vote on the government's position with Cook now expected to lead a huge revolt.

The government may be forced to rely on Conservative votes to secure a majority backing force.

It is thought that Cook will be joined by international development secretary Clare Short.

She made no comment to journalists as she left the Cabinet meeting at 5.30pm.

A spokesman later said she was "reflecting overnight" on her position.

Shadow foreign secretary, Michael Ancram, called on the prime minister to end the rift between his ministers.

"While we understand Robin Cook's reasons for resigning, disunity within the Cabinet is a matter of concern at this time," he said.

"We hope the prime minister will take further steps to ensure that his Cabinet speaks with one voice."

Liberal Democrat Commons spokesman Paul Tyler said the government had lost "its most effective Commons performer".

"His leadership of the Commons gained him an unrivalled reputation amongst fellow members," he said.

"I have no doubt he will be a formidable presence on the backbenches."