The government is aiming to begin a phased withdrawal of troops from Iraq as early as the middle of next year, the Guardian has learned. Work on an exit strategy is at an advanced stage and there will be a significant change of approach by the government after the Iraq election on December 15.

With the first proper election in Iraq completed, Tony Blair and George Bush will be able to claim they have introduced democracy, making it easier for ministers and officials to begin talking openly about withdrawal.

Instead of the government mantra about "staying until asked by the Iraqis to leave", ministers and officials will make it clear that Britain is determined to leave next year if the insurgency allows. Initial discussions between the British and Iraqi governments about a timetable have already taken place. Full negotiations are scheduled to start as soon as possible after a new Iraqi government is formed.

A joint statement from the Iraqi and British governments is to be published, setting out plans for handover of responsibility to the Iraqi army. Britain has about 8,500 soldiers in Iraq. The British intention is to have a phased withdrawal, province by province, with Maysan, north of Basra, likely to be first. The British government insists it will not go without the US. Although US forces are under daily attack, there are parts of its sector quiet enough to allow for handovers too.

British military commanders and defence officials confirmed yesterday that a significant number of troops could be withdrawn from Iraq next spring. They point to May when a new British brigade is due to replace the one which recently took over control of south-eastern Iraq.

Next May is also the month when around 4,000 British troops will be deployed to Afghanistan to take over the international peacekeeping force and replace US troops in the hostile south of the country.

Though military chiefs have told John Reid, the defence secretary, that Britain's Afghan commitment does not depend on a rundown of British troops in Iraq, there is no doubt the beginning of a withdrawal would help the overstretched army.

Mr Blair discussed the exit strategy on Monday with Adel Adbul Mahdi, the Iraqi vice-president who is likely to become prime minister after the election.

A British official said 3,000 could be withdrawn from Iraq without affecting the operational efficiency of the force.

Jalal Talabani, the Iraqi president, said at the weekend that all British troops could be out by the end of next year. Mr Reid was more cautious, suggesting that withdrawal could begin "by the end of next year".

Defence officials say the exit plan is based on extremely complex mix of political and security judgments. "There will be a balance between an operational risk and a political risk," one said. Political pressure might lead to a quick rundown which could compromise the safety of both Iraqi forces and the remaining British troops, officials say.

But British defence officials are also deeply concerned about the danger of a "dependency culture" developing in Iraq. "Only by pushing the case will you get real reform [of the Iraqi security forces]. And they may then stand on their own two feet. It is a complex judgment," said an official.

Impatience with the war also took new political shape in the US, where the Senate demanded that the Bush administration come up with a clearer strategy for ending the occupation. Senate Republicans rejected a Democratic call for a timetable for withdrawal, but passed a bill of their own that requires the White House to deliver comprehensive quarterly reports to Congress on progress towards US disengagement, and calls for Iraqi forces to take the lead in fighting the insurgency by next year.