Defence Secretary Geoff Hoon is to make a statement in the Commons on Monday and will stress any decision would be entirely operational.
The comments come as thousands of people march in central London calling for the withdrawal of British troops.
A Downing Street spokesman said there was “no political dimension” to any redeployment and no link to the US elections.
It is thought the US request is for a UK force of about 650 to step in because a US unit has been earmarked for “combat operations” in the insurgent stronghold of Falluja.
