David Cameron has insisted that the withdrawal of British combat troops from Afghanistan by 2015 was a "firm deadline" that would be met.
Speaking at a Nato summit in Lisbon, the Prime Minister said Afghan forces would begin taking charge of security from early next year.
That process would be complete by the end of 2014, he added, allowing British troops to step back from combat roles by 2015.
"The commitment we have entered into today to transfer the lead responsibility for security to the Afghan government by the end of 2014 will pave the way for British combat troops to be out of Afghanistan by 2015," he told an end-of-summit press conference. "This is a firm deadline that we will meet."
Mr Cameron's comments came after President Hamid Karzai and Nato Secretary General Anders Fogh Rasmussen signed an agreement for the "irreversible transition" to Afghan control by the end of 2014.
Mr Rasmussen stressed, however, that the final handover depended upon improvements in the capability and capacity of Afghan forces, saying that Nato would not leave behind a "security vacuum" which could allow al Qaida to return.
Mr Cameron dismissed suggestions that there were differences between his and Mr Rasmussen's comments on withdrawing from Afghanistan.
"I don't accept that there is a difference because Nato is saying that between early 2011 and 2014 transition will take place and by 2014 what you will have is the Afghans in lead security control throughout Afghanistan," he said. "When I set this deadline I set it beyond that mark."
Asked whether he would pull British troops out while US forces went on fighting in Afghanistan, the Prime Minister said: "We are working extremely closely with our closest ally and we will go on doing that.
"But as I've said I couldn't be more clear about what 2015 is and what it means."
