Britain's six-year occupation of south Iraq will begin drawing to a close in March, and the last troops will leave Basra by June, a senior defence source disclosed yesterday.

But instead of handing over to Iraqi authorities, the British will be replaced at their Basra airport base by a large force of US troops, who will set up their own headquarters there, the source revealed.

The withdrawal follows months of planning and security assessments by British and American commanders. The timetable is expected to be confirmed by Gordon Brown early in the new year.

The initial rundown will be relatively modest, with the tempo increasing later, defence officials said. "It'll be very gradual, and then a fairly steep reduction," one said. By the end of June almost all the 4,000 UK troops now stationed at Basra will be gone. About 300 will remain at the request of the Iraqis to help set up colleges for officer cadets and senior staff officers, and to train the Iraqi navy.

Equipment, from tanks to tents, will be "tailored down", officials said, indicating a gradual rundown. Most of it will be transported back to Britain, in what has been named Operation Archive. The exception will be aerial surveillance drones and Merlin helicopters, which will go to Afghanistan for use by Britain's troops there.

Brown and John Hutton, defence secretary, have expressed the hope Britain's mission in Iraq will have been "fundamentally changed" by the middle of 2009.

However, this is the first time defence sources have put flesh on the withdrawal. It is now clear a crucial factor is the agreement by the US to take over Basra airport with several thousand troops. They will support Iraqi forces and protect convoys bringing supplies from Kuwait.

The US deployment reflects Washington's concern for the region's stability. The US base also reflects the Iraqi government's concern for the vulnerability of its security forces, and any possibility of renewed attacks by Iranian-supplied militia.

Before the British start leaving, regional elections planned for next month will have taken place. British commanders also hope that, by then, senior Iraqi army and police officers will have an effective command, planning, and operations centre.

British officials made clear yesterday they believe the Iraqis are capable of taking the lead in security operations.

Defence chiefs are concerned that the legacy of the British military presence in Iraq should be seen as positive, even though they readily admit that earlier this year the UK did not have sufficient numbers of troops to defeat criminal gangs and militia groups, and restore relative calm in Basra, without US help.

British troops at the airport have not been attacked for two months, officials said. The Mahdi army appeared to have left Basra on the instructions of their leader, the cleric Moqtada al-Sadr. Commenting on the prospect of any renewed threat to Iraqi forces from across the border, the senior British source said: "It is not in Iran's interest for Iraq to be unstable."

For British forces to continue to have a legally binding mandate, the Iraqi parliament will have before the end of this year to ratify a status of forces agreement for UK troops. Defence officials said they were confident that Iraqi MPs would agree later this month to such a deal, which is modelled on that agreed, after months of wrangling, with the US for its troops.

Air Chief Marshal Sir Jock Stirrup, the chief of defence staff, said last month that British troops cannot simply be transferred "one for one" from Iraq to Afghanistan. "It is crucial that we reduce the operational tempo for our armed forces," he said.

Both the training and the family life of British troops have been seriously disrupted by the extent of operations in Iraq and Afghanistan, and defence chiefs are unlikely to back any significant increase in the number of troops deployed to southern Afghanistan until well into next year.

British planners are discussing the scope of operations in Afghanistan with their US counterparts in anticipation of a large increase in US troops next year.

Stirrup said last week that it was not enough just to deploy more British troops to Afghanistan. What was needed was help in building the economy and civil society. "I and others have been saying for over two years now we have to get a grip of the civilian effort," he told the Royal United Services Institute.