The UK is preparing to pull 1,500 troops out of Afghanistan next year before rapidly accelerating its withdrawal at the beginning of 2014.
Ministers are expected to endorse plans that stick closely to advice from British commanders in Helmand, and from Nato headquarters in Kabul, when they come before the National Security Council (NSC). The strategy would leave the bulk of UK forces in place for most of next year, though there is also an option for a "fast-track" pullout by spring 2014.
Pressure to speed up the withdrawal has intensified because of recent atrocities against Afghan civilians, which provoked protests across the country, and left troops in Nato's International Security and Assistance Force (Isaf) vulnerable to retribution.
A Royal Marine and a soldier from the Adjutant's General Corps were killed on Monday by a member of the Afghan security forces, raising further questions about the viability of the Nato mission.
But according to sources in Whitehall, ministers have been persuaded – so far – that a greater folly would be to leave Afghanistan before local forces are ready to lead the fight against the Taliban.
Only 500 British troops will leave Afghanistan this year, leaving 9,000 still in Helmand at the start of next year.
Under current plans, the force will be reduced in size again in September 2013, bringing the total down to 7,500. The military intends to speed up the withdrawal markedly after that, with planners drawing up two options.
The first – and preferred – scenario involves withdrawing another 6,000 troops before September 2014, leaving a rump of 1,500 in Afghanistan at the time of the formal handover to Afghan security forces at the end of that year. Most of them would leave the country in early 2015.
A more radical option involves withdrawing all but 1,500 troops from Afghanistan by April 2014. In practice, this would mean the British brigade withdrawing from Afghanistan in the spring being replaced by a much smaller force.
"The option we take depends on the US," said a Whitehall source. "If the Americans increase the size and speed of their withdrawal then we may have to consider a much quicker exit in early 2014."
During David Cameron's recent visit to the White House, both he and Barack Obama spoke about transferring security responsibility to the Afghans in 2013.
Officials say this was misinterpreted as a signal that more troops would return early, but that is not what the military wants.
"Even if more troops came home next year, we'd have to support the Afghans in different ways with more military advisers," said one.
"If you ask Afghans to take the lead before they are ready, then you still need to support them, so the overall numbers of British personnel in the country might not change..
"Transferring the lead on security to the Afghans early gives something to the Afghans, the French, and the American audiences, but it won't change much."
Obama is due to speak at the Chicago summit on Afghanistan in May, six months before he seeks re-election.
"We expect Obama to keep something up his sleeve in May," said a Nato commander.
"A lot will turn on what he says then, and what happens in November. But so far the British government is not saying anything to us to suggest they will not see this through."
The Ministry of Defence said no decision would be taken until the NSC reviewed and rubber-stamped the strategy.
"No decision has yet been taken about the drawdown of UK forces," said an MoD spokesman. "As the prime minister has already stated, ours will be a steady and measured drawdown leading up to 2014.
"Both he and President Obama confirmed in Washington recently that our transition plan is on track, realistic and achievable."
Last week Cameron told the Commons the UK would "not be in a combat role in Afghanistan after 2014, nor will we have anything like the number of troops that we have now".
He added: "What I discussed with President Obama in America is making sure that in 2013, if there are opportunities to change the nature of the mission and be more in support rather than a direct combat role, then that's something that I think everyone will want to see."
Giving evidence to MPs on Monday, the new national security adviser, Sir Kim Darroch, said that "at some point in the middle of 2013 all the different provinces of Afghanistan will transition to Afghan lead".
A poll in Tuesday's New York Times reflects how support for the US effort in Afghanistan is dropping sharply. US soldiers have been responsible for burning the Qur'an, and been filmed urinating on dead Afghans in recent weeks. A US staff sergeant, Robert Bales, shot 17 Afghan civilians earlier this month.
The poll of 1,000 showed 69% thought that the United States should not be at war in Afghanistan, up 16 points from February.