Defence Secretary Geoff Hoon says Britain will have failed in its duty as an ally if it does not agree to a request from America to relieve US troops in more dangerous areas of Iraq.
British troops have secured local goodwill in southern Iraq
The US wants back-up in an area nearer to Baghdad and a final decision is expected later this week.
Mr Hoon has rejected claims that the request is political and an effort to boost George W Bush's election hopes.
He said it was designed to free up US forces for operations in other areas.
There was a "very clear operational justification" for the request which had been received on October 10, he added.
"We want to make clear that the request is a military request and although it is linked to elections it is not linked to the US elections," Mr Hoon told MPs in a Commons statement.
Instead it was about creating the "best circumstances" for the Iraqi elections to go ahead in January.
"The government remains totally committed to holding free elections in January" and "seeing Iraq take up its rightful place in the international community," he said.
"The US request is for a limited number of UK ground forces to be made available to relieve US forces to allow them in turn to participate in further operations elsewhere in Iraq to maintain the continuing pressure on terrorists.
"The request does not ask for British troops to be deployed in Baghdad or Falluja," Mr Hoon added.
Speaking on the BBC's Newsnight, he denied a decision had already been made, but said the request "seems to me to be part of the responsibility of a good ally to support others in that situation".
He said he understood public concern at British troops being moved to more dangerous areas, but insisted that "overwhelmingly what people want to see [is] security restored to Iraq".
Mr Hoon would not confirm that the troops would come from Scottish-based Black Watch, but acknowledged that it was the UK's reserve battalion.
"They are there in the event of there being a serious breakdown in law and order that requires their participation.
"There is no serious breakdown. Then they can be spared for these extra kinds of operations should the military advice be to that effect."
Once all the issues had been considered, the chief of the defence staff was expected to recommend whether to comply by the middle of this week.
Shadow defence secretary Nicholas Soames warned that any such deployment would leave "a big capability gap" in the south-eastern British sector and would represent "a fundamental change" in UK operations in Iraq.
"Given that the security situation could well deteriorate between now and the Iraqi elections, how do you plan to fill such an important capability gap?" he asked.
Black Watch members had been told last week that they would not be coming home, he said.
The families of members would have "watched with disbelief the unfolding shambles of the last few days," Mr Soames added.
BRITISH FORCES IN IRAQ
9,200 troops deployed to the Gulf
1,400 of those are reservists
Most troops in Basra and al Muthanna provinces
1 Mechanised Brigade is currently 'lead formation'
6,315 troops from 10 nations also serve in the area
For the Liberal Democrats Paul Keetch urged the government to set out what the command structure would be and what input British commanders would have in setting out tasks for UK troops.
Earlier Lib Dem leader Charles Kennedy warned against Britain "allowing itself to be sucked further into the mire in Iraq".
As the election neared, Mr Kennedy said, the Bush administration would be "looking for some major breakthrough in terms of their Iraq campaign, and that they want to divert forces accordingly" .
But most anger was felt on Labour's backbenches, with Glenda Jackson MP accusing the government of providing "mercenaries for a Republican army".
Bolsover MP Dennis Skinner said the government was handing George W Bush a "lifeline" and an "oxygen cylinder" by freeing up American troops for a pre-election offensive.