George Bush was left increasingly isolated and under pressure last night after Britain confirmed plans to withdraw 1600 soldiers from Iraq.

The first UK troops to be sent home could arrive as early as this time next month, the Ministry of Defence indicated. Following Britain's lead, Denmark and Lithuania said they would withdraw most of their troops from Iraq by August.

Many other countries which joined the US-led operation, such as Japan and Spain, have already pulled out their troops and others, such as South Korea, have announced plans to do so. Mr Bush, by contrast, is sending 21,500 more troops to Iraq, where there are around 141,000 American forces.

Britain will still have around 5000 soldiers for the foreseeable future, while Australia, another key US ally, said it had no immediate plans to reduce its 1400-strong force in and around Iraq.

Denmark is withdrawing 470 soldiers in southern Iraq, serving under British command, and replacing them with a small helicopter unit of 55 soldiers. Lithuania is planning to pull out its 53 troops, who have served under Danish comand.

Democrats in Washington seized on the news to pressure Mr Bush to bring troops home.

However, US vice-president Dick Cheney said Tony Blair's announcement was "an affirmation of the fact that in parts of Iraq things are going pretty well".

Mr Cheney said Washington would not back "a policy of retreat". He added: "We want to complete the mission, we want to get it done right, and we want to return with honour."

The Democrats, who passed a non-binding resolution in Congress last week opposing the US troop increase, said the British decision underlined the need for a policy rethink on the four-year battle, in which 3148 American soldiers have died.

Senator Edward Kennedy, a Massachusetts Democrat, said: "It is long past time for the administration to change course, begin the hard work of diplomacy with Iraq and its neighbours, and redeploy our forces."

There was also concern about the effects of the British pull-out on the ground in Iraq. Anthony Cordesman, at the Centre for Strategic and International Studies in Washington, said the move would play into the hands of Iranian-backed Shi'ites. "The British cuts will in many ways simply reflect the political reality that the British lost' the south more than a year ago," he said.

In Basra, Iraq's second city and main centre for oil production, many expressed relief, but some voiced fears that the withdrawal was premature. "They were occupiers and they should have left long ago," said Nour Abdul-Muttalib, 29, a teacher.

Jaafar Saleem, 38, a businessman, said: "I think their exit will produce lawlessness in the city."

In London, the MoD spokes-man said the drawdown of soldiers would start from "spring", which begins on March 21. He said the regular rotation of troops was due to take place during "March and April" and gradually the 7100 troops would be replaced by 5500 new ones.

In terms of timing, this will mean the troops would leave Iraq as the campaigns for the Scottish parliamentary, Welsh assembly and local council elections get into full swing.

Among those serving in Iraq are the 51st Highland, 7th Btn The Royal Regiment of Scotland, and a company of The Black Watch, 3rd Btn the Royal Regiment of Scotland.

Defence Secretary Des Browne is today due to set out the operational details of British forces' future deployment in and around Basra.

Prince Harry will learn whether his regiment, the Blues and Royals, will be deployed.

Reports have suggested it will form part of a Household Cavalry detachment as 1 Mechanised Brigade replaces 19 Light Brigade.

It is not yet known, though, whether the third in line to the throne will become the first royal to serve in a war zone since Prince Andrew, his uncle, flew helicopters during the Falklands conflict in 1982.

Yesterday in his Commons statement, Mr Blair held out the possibility of further withdrawals to leave a total of fewer than 5000 by late summer. However, he explained British troops would support local authorities into 2008 "for as long as we are wanted and have a job to do".

"What all this means is . . . the next chapter in Basra's history can be written by Iraqis."

Tory leader David Cameron welcomed the withdrawal but said the security situation in Basra remained "dire". Sir Menzies Campbell for the Liberal Democrats called on the government to set a timetable for a full withdrawal.

Alex Salmond dismissed Mr Blair's "self-justification" for the "horrors of the last four years" and called on him to apologise for the misinformation about weapons of mass destruction in Iraq.