LONDON — British Prime Minister Gordon Brown told lawmakers Thursday that Britain will withdraw almost all its forces from Iraq by July, leaving fewer than 400 troops to complete a naval training mission.

Following a one-day visit to Iraq Wednesday, Brown told the House of Commons that the bulk of the country's 4,100 troops will be pulled out of southern Iraq beginning May 31.

Brown first announced the withdrawal in Baghdad after talks with Iraqi Prime Minister Nouri al-Maliki, but told lawmakers Thursday that a few hundred U.K. personnel will remain in place to continue training duties.

"In the last five-and-half years, Iraq has faced great challenges and endured dark days. But it has also made very significant progress," Brown told lawmakers. "We can be proud of the way our forces carried out their mission."

Brown declined to authorize an immediate inquiry to examine mistakes made in the run-up to the U.S.-led 2003 invasion of Iraq and errors in postwar planning.

"I have always said this is a matter we will consider once our troops have come home. We are not at that stage," Brown said. He has previously said that Britain will eventually hold a formal investigation into the Iraq war.

Brown's office declined to specify whether an inquiry could be called after July, when the bulk of Britain's forces will be home, or only following the conclusion of the naval training mission.

Britain's troops are stationed mainly at a camp close to an airport in the southern city of Basra, Iraq's second-largest urban center. The U.K. provides the second-largest military presence in Iraq after the United States.

At the height of combat operations in March and April 2003, Britain had 46,000 troops in Iraq. A total of 178 servicemen and women have died there since 2003.

Brown said a memorial at the Basra air base to the dead servicemen and woman would be removed and relocated to a "fitting resting place" in Britain.

Britain's defense ministry said U.S. troops will take over Britain's base once U.K. forces leave, sending soldiers to Basra to secure military supply lines which pass from Kuwait through southern Iraq and on to U.S. bases.

Brown said the remaining British forces will consist of around 400 mainly naval personnel. They are based in the port town of Um Qasr, south of Basra and close to the Kuwait border, according to the defense ministry.

The forces are focused on training Iraq's navy to defend oil platforms stationed off the coast, which are seen as key to the revival of Iraq's economy.

Opposition lawmakers on Thursday urged Brown to immediately authorize an independent inquiry into the war.

Liberal Democrat Party leader Nick Clegg said Brown's government must not be allowed to end the conflict as he claimed it had begun "in secret, unaccountable and behind closed doors."

Britain has previously held an inquiry into the death of David Kelly, a British government weapons scientist who killed himself in 2003 when he was exposed as the source of a British Broadcasting Corp. report that accused Tony Blair's Downing Street office of "sexing up" pre-war intelligence.

A separate 2004 inquiry looking into intelligence on Iraq did not fault Blair's government, but criticized intelligence officials for relying in part on seriously flawed or unreliable sources.

But government critics say a single, independent inquiry to look at all aspects of planning for the invasion and its aftermath is necessary.