Britain could start withdrawing most of its remaining 4,100 troops from Iraq from March next year due to the improving security situation, a defense source said on Wednesday. The withdrawal could be over by June, six years after former British Prime Minister Tony Blair sent 45,000 British troops to help the United States invade Iraq to topple Saddam Hussein, the source added. The defense ministry in London confirmed there would be a "fundamental change of mission" in Iraq early next year, although it would not give a date. "Our position remains that we will judge it on military advice at the time, but there has been significant progress," a spokeswoman said. The defense source said the withdrawal would free up some assets that could be sent to Afghanistan, although military chiefs and analysts say British troops will not be immediately redeployed in great numbers. In a speech earlier this month, Jock Stirrup, overall commander of British forces, spoke of the need for a pause between operations to avoid stretching resources. "We cannot simply make a one-for-one transfer from Iraq to Afghanistan," he said. Britain currently has about 8,000 troops in southern Afghanistan. Security in the country has deteriorated in the past year in the face of a Taliban insurgency. Defense analyst Paul Smyth, from the Royal United Services Institute, a London-based think tank, said troops needed a break between missions to rest and retrain. "It's going to take time, it's not immediate," he added. "We expect to move from next year toward a long-term, broad-based bilateral relationship with Iraq similar to the relationship we have with other allies in the region, including a training and education role for our military personnel," it said. It also underlined that any withdrawal would be subject to conditions on the ground: "British forces will only leave southern Iraq when we are confident that the Iraqis can operate effectively without our support." U.S. General David Petraeus said this week that violence in Iraq in the past few weeks had fallen to its lowest level since mid-2003 and that security gains, while still at risk of reversal, were less fragile than before.