Tony Blair set the clock ticking for British troops to start withdrawing from Iraq in six months’ time as the United States announced a withdrawal of two combat brigades from the country.

The news came as it emerged that more than 1,000 British soldiers had already been pulled back from frontline duties.

Troops in half the area under British control have stopped routine patrols and have returned to their barracks, The Times has learnt.

Senior defence sources said that the 800 British troops in Maysan province and 300 in Muthana province had switched to a "tactical overwatch" role — remaining in their barracks and only going out on patrol with the Iraqi security forces when they asked for help. This is the first stage in withdrawing altogether from the provinces, following a similar pattern adopted in Northern Ireland.

The tactics were disclosed shortly before Donald Rumsfeld, the US Defence Secretary, announced that President Bush would reduce the American force in Iraq by up to 10,000 troops early in the new year.

Speaking on a surprise trip to Iraq, the second visit by a senior American official this week, Mr Rumsfeld told soldiers at a base in Fallujah today that the US would not be deploying two combat brigades scheduled to arrive in the country next year.

"President Bush has authorised an adjustment in US combat brigades in Iraq from 17 to 15," said Mr Rumsfeld, who declined to specify the exact size of the reduction. American combat brigades consist of between 3,000 and 5,000 troops.

"The adjustment being announced today is a recognition of the Iraqi people’s progress in assuming added responsibility for their country," said Mr Rumsfeld.

Earlier this year, the Pentagon set the size of the US force in Iraq to a "baseline" of 138,000 troops. The deployment was temporarily increased to 160,000 during the recent general elections.

Mr Rumsfeld said a further withdrawal would be discussed once the new Iraqi government was assembled in the spring: "We anticipate future coalition force-level discussions at some point in 2006, after the new Iraqi government is in place and is prepared to discuss the future," he said.

The US Defence Secretary began his eleventh visit to Iraq since the US-led invasion of the country in 2003 just as the British Prime Minister was preparing to leave.

On his own surprise visit, Mr Blair told British troops yesterday that the situation in Iraq seemed transformed compared with his last Christmas trip to Basra. Speaking from a flat-bed trailer, he said that progress was under way to "eventually draw down our capability". He appeared to confirm a suggestion from a senior general that significant withdrawals could begin in six months.

Asked directly about this, Mr Blair said: "There is no reason why, if everything goes to plan, then this is our strategy. We want to draw down our forces, we don’t want to keep people longer than we need to, it is happening."

Major-General Jim Dutton, who has just completed his tour as commander of the Multinational Division Southeast, based in Basra, said that it was realistic to expect withdrawals by June.

Top American and British military and diplomatic figures told the Prime Minister yesterday that security conditions would be sufficiently benign to merit significant troop cuts by that time.

The Times has learnt that the chiefs of staff at the Ministry of Defence are due to meet in March to decide on troop deployments to Iraq with a view to recommending cutbacks in at least two of the four provinces in southern Iraq.

The meeting to fix a "statement of requirements" for Iraq is expected to call for troop withdrawals from Muthana and Maysan provinces, and a small reduction in force levels based in Basra province. This could be timed to when the next changeover in troops is due in May, with 7th Armoured Brigade being replaced by 20 Mechanised Brigade.

Optimism about security conditions in southern Iraq was echoed by General George Casey, America’s top commander in Iraq, who flew to Basra to see Mr Blair. He said that Iraqi forces would be ready to take over 75 per cent of duties in some parts of the US sector of the country by the summer.

General Abdul Latif, commander of Iraq’s 10th Division in southern Iraq, which is being trained by the British military, has said that his unit of around 8,000 troops would finish training by the end of March.

Mr Blair said: "British troops have a very high regard for the Iraqi Army, and those working with the police are finding they are good to work with as well. That is hopeful. The troops here I have talked to were rather more upbeat about Iraqi capability than I expected."

Mr Blair brushed aside suggestions from Sir Jeremy Greenstock, the former British Ambassador to Iraq, that large numbers of British troops could be in Iraq for five years or more. Mr Blair’s spokesman added: "Sir Jeremy Greenstock has not been here for some time and the picture I got today was very different."

Mr Blair, rejecting body armour and opting for a blue blazer and chinos, conducted a walkabout of British troops before addressing two groups, each of around 200 soldiers, thanking them for their service.

However, although Mr Blair indicated a dramatic cut in troop numbers from the 8,000 now in Iraq, he also indicated that a small contingent might have to stay a very long time. "We still have troops in Bosnia today (770), but it is not the same number as ten years ago."

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