Thousands of British troops will start serving longer tours in Afghanistan, the defence secretary has announced.
From October, soldiers will be deployed for up to eight months instead of six, rising to up to nine from next year.
Up to 3,700 personnel could be affected by the move, which means troops will remain in Afghanistan into 2015 - after combat operations have ended.
The announcement prompted warnings from Labour, veterans and military families of the possible toll on those serving.
Defence Secretary Philip Hammond said fighting was expected to end by 2014 but that a "relatively small number" of British troops could stay for tours of up to nine months.
Speaking in the Commons he said the Ministry of Defence had been looking at how best to "deploy what will be declining numbers of troops and smaller amounts of equipment".
'No policy change'
"After more than a decade in which fighting the insurgency has been a primary focus," he told MPs, "The campaign is changing and the UK's military role in Afghanistan is evolving from combat to one of training, advising and assisting the Afghans."
Elfyn Llwyd, Plaid Cymru's Westminster leader who voted against the original invasion in 2001, said he was pleased combat was coming to an end but warned the tours could lead to "loss of life".
"History shows that during periods of drawdown it's especially dangerous and there will be an increased number of people suffering conditions such as PTSD [post-traumatic stress disorder] and associated problems."
The defence secretary said the announcement would not affect future troop deployments and was "emphatically not a systematic shift in policy".
He said it was not possible to be precise about the number of British troops who would be affected by Tuesday's announcement.
But "current estimates suggest between 2,200 and 3,700 overall may deploy more than six-and-a-half months", he said.
The mother of one soldier who has been told to expect an eight or nine-month tour said it would make life much harder for those serving and their families.
"He loves his job, but he's not happy about the length of time," said Jacqui, whose son was told about the extended deployment a month ago.
"When my son told me he was going on a longer tour my heart sank. It's so hard trying to remain positive and strong as it is without increasing the length they will be away."
And one veteran's charity called for the longer tours to be properly managed.
"Long deployments require significant post-deployment respite and stability if they are not to have a detrimental long-term effect," said Combat Stress chief executive Andrew Cameron.
Mr Hammond also announced that service personnel touring for more than seven-and-a-half months would be paid an extra allowance of £50 a day - a move dismissed by the mother of two sons in the army.
Sharon Huntley, who has one son serving in Afghanistan and another who recently returned, said: "No amount of money can compensate for what they see and experience over there."
Responding to the government's announcement, Labour said they wanted the extension to be successful.
"We see the logic in the government's move but many will be concerned about the impact on the individuals affected," shadow defence secretary Jim Murphy added.
Liam Fitzgerald Finch, who served in the Army for 12 years and was awarded the Queen's Gallantry Medal for defusing bombs in Helmand, said there would be "tangible mental health issues" for the soldiers concerned. He now works with Felix Fund, a charity which supports bomb disposal experts and their families.
"It properly ages you," he said. "The tempo the guys work at is ludicrous but all [the government] are looking at are the costs - not the impact on the individual."
British forces have been mainly based in Helmand province in southern Afghanistan after operations began in 2001.
Most international troops are scheduled to leave Afghanistan by the end of 2014.