BASRA, Iraq (AFP) - British forces officially started to withdraw from Iraq on Tuesday, signalling the end of six years of military operations that began with the US-led invasion that ousted Saddam Hussein. The flag of the British headquarters in Basra was lowered during a solemn ceremony marking the start of a months-long process that will see all 4,100 British troops leave the violence-wracked country by the end of July. One hundred and seventy-nine gallant warriors of the United Kingdom made the supreme sacrifice, said General Ray Odierno, the senior US army officer in Iraq , in tribute to the British personnel who have died in the country since 2003. We have shed blood together and that is a bond that no man can break, he told about 300 guests, including British and Iraqi officers and diplomats, shortly before a US army flag was raised and the colours of the Royal Marines were lowered. You have restored hope where chaos reigned, Odierno added. Britains chief of defence staff Air Chief Marshal Sir Jock Stirrup paid tribute to those who made the ultimate sacrifice and described the Iraq campaign as a noble endeavour. This occasion is about the countless number of men and women from the far-flung corners of the United States and from Great Britain who have striven with such courage here, he said. Major General Andy Salmon, the senior British officer in Basra, handed over the southern base to an American commander, in a key step towards all foreign troops leaving the country and a full return to Iraqi sovereignty. The colours of the coalitions Multinational Division South-East, a specially inscribed Royal Marines flag, were lowered and replaced with the standard of the US Armys 10th Mountain Division. Salmon, a Royal Marine and veteran of the 1982 Falklands conflict and 1991 Gulf War, was among 40 British troops to leave Basra after Tuesdays ceremony. He and his predecessors were praised by Iraqi army General Nassir Abadi. Now is the time to thank those British personnel who have served in our country and the many who have contributed so much, Abadi said. We owe a debt of gratitude that we can best repay by continuing the progress that my country has made. In London Blairs successor, Gordon Brown, paid tribute to the British troops. There have been difficult times along the way, but British troops have made an outstanding contribution to laying the ground for a stable and increasingly prosperous Basra part of a stable, secure and prosperous Iraq , Brown said in a statement. Tuesdays departure began almost 50 years after Britains previous exit from Iraq , in May 1959, when the last soldiers left Habbaniyah base near the western town of Fallujah, ending a presence that dated back to 1918. A Royal Navy training team is based at the southern port of Umm Qasr, and its role is expected to continue although a new agreement has yet to be reached. The British pullout comes as the US military also steps up preparations to leave Iraq . Under a US-Iraqi security agreement signed last November, American troops must withdraw from major towns and cities by June 30 and from the whole country by the end of 2011.