The chair of the Defence Select Committee has told Forces News that it is “fanciful” to believe British forces could stay in Afghanistan if the US pulls out.
Last month reports emerged that President Trump is considering withdrawing 7,000 American troops.
That is half of all US forces in the country.
The conflict has been continuing for 17 years and at one point saw 100,000 troops on the ground.
It has proven to be a difficult war for America but one that Donald Trump vowed he would end during his Presidential campaign.
In 2017 he sanctioned a troop surge at the request of then-Defense Secretary Jim Mattis.
But in the last two weeks things have changed, first the news that 2,000 US troops would return from Syria and then reports that the President is considering a similar move in Afghanistan.
The news shocked Washington and America’s allies and Defense Secretary James Mattis, a former Marine, walked out.
Britain has around 1,000 non-combat troops in Afghanistan in several roles; providing security around Kabul and training and advising at the Afghan National Officer Academy outside the city.
The withdrawal of the US will have an effect on the UK forces that remain there.
Dr Julian Lewis MP, Chairman of the Defence Select Committee, says:
"The idea that Britain and other smaller powers would carry this on by itself if America pulls out is pretty fanciful.
"I think that British troops would be considerably effected, in terms of their future deployments, if they could no longer rely on our strongest ally in taking part."
The White House says the President hasn’t issued any order to withdraw US troops, but they’ve also not denied the reports are true.
Squadron Leader Andy Rolston, a spokesman for NATO's Resolute Support in Afghanistan said there are no plans for a British drawdown.
For the US military, the Afghan campaign has been costly.
More than 1,800 American troops have been killed in action there since 2001, with more than 20,000 wounded on the battlefield.
The majority of western combat forces left the country in 2014.
Despite record losses, NATO commanders say the Afghan military IS now strong enough to stand alone, even after a potential US draw-down.
In recent weeks US officials and representatives of the Taliban met again in the UAE, their third meeting to try and find a settlement to the war.
After 17 years and close to a trillion dollars of US spending the Taliban remain a potent force, controlling much of the country.
It is unclear what effect the US pullout would have on the peace process and her NATO allies on the ground, but, if the reports are true the UK military may well find itself reassessing its own position in Afghanistan too.