Tony Blair has been challenged to put the UK's decision on a US request for back-up from British troops in Iraq to a vote of MPs.
There are morale worries for the Black Watch
Liberal Democrat leader Charles Kennedy joined 44 Labour MPs in saying there should be a Commons test for the move.
In the Commons, Mr Blair insisted no decision had been made on sending troops to a new part of Iraq and said it would depend on army chiefs.
The Tories asked why the mooted troops move was necessary.
It is thought redeploying the British soldiers to the area south-west of Baghdad would free US troops for an assault on Falluja.
'Valley of death'
In a Commons motion, the 44 Labour MPs say the move would "significantly increase the risk" to British troops.
Mr Kennedy said he would not support any new troop deployment which went further than helping British soldiers carry out their current role.
He asked: "If the prime minister in due course is confident of the case for acceding to the request the Americans have made, will he then come to test his arguments and put it to the House for a decision?"
Scottish National Party leader Alex Salmond later said it was right for MPs to decide whether Black Watch soldiers were sent to a "valley of death" in Iraq.
The prime minister did not answer the vote calls directly but said he would not politically overrule advice from military commanders.
Mr Blair said if the troops were redeployed they would remain under British military command, and that, whatever was decided, Black Watch soldiers would return to the UK by Christmas.
Conservative leader Michael Howard said he did not support calls for a vote.
But he asked why it was necessary to redeploy 650 British troops when there were 130,000 US troops in the country.
Mr Blair said not all American forces were right for the job and he rebutted claims the request was in any way linked to the US presidential elections.
"I don't believe our commanders... would want the British troops to do this unless they would it was necessary for the achievement of our overall aims," he said.
He criticised ill-informed speculation that the troops could be sent to Baghdad or Falluja.
But he could not give more details, saying it was not right to advertise troop moves to terrorists.
Mr Blair also paid tribute to the work of charity worker Margaret Hassan, who is being held hostage in Iraq.
Earlier, Downing Street refused to discuss details of the timetable for the possible deployment, insisting the decision would be made in the normal way.
That has led to some speculation that it may be delayed until after the US Presidential election.
Despite ministers' assurances, the Daily Telegraph quotes unnamed defence sources saying a decision was made more than a week ago.
A military reconnaissance team visited the proposed new operations area on Tuesday and has yet to produce its full report.
The general insisted the UK could refuse the American request.
But he said: "I don't think it would be militarily sensible to do so.
"The request is in response to the situation on the ground. There's been a spike in insurgent activity as a result of the Ramadan period."
'Weeks not months'
General McColl said other issues would be discussed in London, including concerns for the morale of soldiers of the Black Watch.
British commanders in south-east Iraq had been consulted about the possible effects the potential gap the move could create in the region near Basra.
"The view is that this is an acceptable risk," said General McColl, who suggested the deployment was likely to last "weeks not months".
But Air Marshal Sir John Walker, ex-deputy head of the Joint Intelligence Committee, told BBC Radio 4's World At One: "If we get engulfed in the American heavy-handed approach up in Baghdad we could lose a lot of the good will that we picked up in the very good work that was being done in Basra."
The Commons defence select committee is asking Defence Secretary Geoff Hoon to appear for questioning about what its chairman called a "highly contentious decision".
Nearly 7,500 British troops are currently serving in southern Iraq, based mainly around the port of Basra.