Defence Secretary Geoff Hoon has paved the way for British troops to be switched to different areas of operation in Iraq to fill in for US combat groups taking part in a blitz on terrorist groups.

Last night, Mr Hoon indicated that troops from the Black Watch regiment would be involved.

They are currently the reserve force covering the British area of responsibility around Basra.

Mr Hoon told MPs he expected a final recommendation soon from the Chief of the Defence Staff, General Sir Michael Walker.

He told the Commons that UK forces would not be going to insurgent strongholds in Baghdad or Fallujah, following the US request for assistance.

The Defence Secretary said some members of the Black Watch regiment might have understood informally that they would be coming home before their six-month tour officially ends later this year.

This would not be the case if they were chosen for this latest operation, said Mr Hoon.

Speaking on BBC2's Newsnight, Mr Hoon said: ''They (the Black Watch) are there in the event of there being a serious breakdown in law and order that requires their participation.''

He said the Black Watch was "determined to carry through this operation should it be decided they participate".

Most MPs assume the British group would be sent north from Basra to take the place of US soldiers in an area south of Baghdad.

The Defence Secretary said: "The only relevant fact is that the UK military received a request on October 10 from the US military command for assistance."

He confirmed that would be outside the existing British area of operations, but stressed UK forces had carried out such missions before.

No details such as the timing or length of the operation had been decided, added Mr Hoon.

"I expect the final recommendation from the Chief of Defence Staff by the middle of the week."

At its swiftest, that would enable Thursday's Cabinet to approve the move, with MPs being told later that day.

Defence sources, however, were reluctant to be drawn on timing - saying that could only be decided if, and when, the chiefs of staff had been fully briefed and made their decision.

The Defence Secretary said: ''Speculation has focused on the suggestion that the request is somehow political and its timing linked to elections.

"I want to make clear that the request is a military request."

He said there would be no difficulties with UK forces operating alongside US colleagues.

Anti-war Labour MPs have seized on the possible deployment to intensify their criticism of Prime Minister Tony Blair's decision to back US President George Bush in the conflict.

Some said the move was designed to give Mr Bush a pre-election boost, an accusation denied by No 10.