But that declaration, apparently aimed at a possible general election next month, backfired after Mr. Brown was accused of playing politics. It also turned out that some of the troops he described as being withdrawn were already home, and others had already been announced as heading back.  In his formal statement to Parliament, Mr. Brown said a decision would be made in the spring as to how long the reduced force of 2,500 would remain in Iraq.  At a briefing at the Foreign Office after Mr. Brown’s statement, a senior British official said of the 2,500 troops that “there was no guarantee they will be there beyond the end of 2008.” The official said Britain was on a “glide path” of irreversible reductions. The official spoke on condition of anonymity under Foreign Office rules.  In his response to Mr. Brown in Parliament, the leader of the opposition Conservative party, David Cameron, seemed to sum up the content of Mr. Brown’s statement, saying, “Now the troops are coming home.”  Though Mr. Brown pronounced himself satisfied with the security situation in Basra, others said the British were leaving behind a lawless, violent place.  Toby Dodge, a senior fellow at the International Institute for Strategic Studies in London, said the Iranian influence in southern Iraq would only increase as the British scaled back. The Iraqi police around Basra have been “heavily infiltrated by Islamic militia,” Mr. Dodge said.  In essence, he said, the British had been “driven out by Islamic radicals with nothing more than rocket-propelled grenades and mortars.”  Explaining his rationale, Mr. Brown told Parliament, “The Iraqis are now able to take responsibility for the security themselves.” He described a two-phase process of handing over responsibility for security in Basra Province, where the British have been based since the war’s start.  It would start with British forces training and mentoring Iraqi security forces, securing supply routes to the Iranian border and providing backup to local security forces. In the second phase, starting in the spring of 2008, British troops would retain a more limited ability to intervene by force, the prime minister said.  The troop level would be reduced from the approximately 5,000 now in Basra to 4,500 and then to 4,000, and then to 2,500 by spring, Mr. Brown said.  At the Foreign Office briefing, the senior British official said the drawdown of British troops, including the reduction to 2,500, had been discussed in detail with the American commander, Gen. David H. Petraeus. “It is a number with which General Petraeus is content,” the official said.  The 2,500 British troops in the final phase of the British deployment would be stationed outside the city of Basra at the Basra Air Station, and they would be almost completely involved in instructing the trainers of two divisions of Iraqi soldiers, the official said.  About 500 British troops, serving support roles to those still in Iraq, would be based in a neighboring country, the official said. He indicated that those support troops would be based in Kuwait.  In determining the size of the reduction of the British troops, Mr. Brown has had to deal with three constituencies: the British electorate; the British Army, whose commanders have complained about the Iraq deployment stretching the military too thin; and the White House.  Despite statements by the British that the Pentagon was satisfied, Mr. Dodge said he believed that the “White House is deeply uneasy” about the decision. He cited remarks by Gen. Jack Keane, a retired Army vice chief of staff and an architect of the American “surge,” who has expressed frustration at the disengagement of British forces in Basra.  In his statement to Parliament, Mr. Brown also sought to defuse an outcry over the future of Iraqi civilians who have worked with the British troops. Those Iraqis who have worked alongside the British for more than 12 months would be eligible for aid and emigration to other countries, possibly including Britain, he said.  At a news conference at 10 Downing Street before his statement to the House of Commons, Mr. Brown was pummeled with questions about why he allowed speculation over an election announcement to reach a fever pitch before backing away from calling an election.  “Yes, I did consider holding an election,” Mr. Brown said. “Yes, I looked at it.”  But in the end, he said he followed his “first instinct” to take more time to show voters his vision for the country, particularly in housing, education and health.  The British news media reported Monday that Mr. Brown probably would have won a November election, but the current majority of more than 60 seats would most likely have shrunk substantially.  The surveys showed the electorate to be volatile after both the Labor Party and the Conservative Party held their conferences.  Mr. Brown said Monday that an election was “not likely” in 2008. He must call an election by 2010.  A version of this article appears in print on , on Page A1 of the New York edition with the headline: Britain to Halve Its Force in Iraq By Spring of ’08. Order Reprints| Today's Paper|Subscribe
The move suggests that the alliance on Iraq forged between former Prime Minister Tony Blair and Washington is unraveling.