British Special Forces are being lined up to remain in Afghanistan after soldiers withdraw, The Telegraph can reveal.

Boris Johnson is expected to make a final decision on Monday at a meeting of the National Security Council.

However, one former SAS soldier who was recently in Afghanistan told this newspaper that the decision was as good as done and a small number of troops will remain in the country as an “advisory group”.

He added the role of the SAS in Afghanistan will be to “provide training to Afghan units and deploy with them on the ground as advisors”.

The former SAS source added there was “no determined time” for how long British special forces will remain stationed throughout the country.

“As long as they continue to see value they will keep forces there. It’s not a pleasant place at the moment, people are scared and rightly so.

“The Taliban control the countryside and are just waiting for the coalition to leave. They are making it abundantly clear at every opportunity that their peace is with the coalition and not the Afghan government. The country will implode.”

However, one senior military source cautioned that the Prime Minister was yet to make an official decision on the matter, saying “It’s our job to provide a range of different options to the Government”.

The war in Afghanistan has outlasted numerous world leaders

General The Lord Dannatt, who earlier this week wrote in this newspaper that the Taliban has “prevailed” and that civil war was “highly likely”, told The Telegraph: “It’s perfectly reasonable to say that given our 20-year commitment to the country, and our aspirations to try and help the Afghan people… there are a number of ways that the British government should and undoubtedly will continue to support the Afghan people.

“If it can do so through supporting their military, through the use of our Special Forces, this is something that would not surprise me.”

A Ministry of Defence spokesperson said: “The UK is involved in ongoing discussions with US and international allies regarding the future of our support to Afghanistan.”

Meanwhile, hundreds of Afghan troops fled into neighbouring Tajikistan as the Taliban’s march through northern Afghanistan gained momentum with the fall of 10 districts in Badakhshan province.

The militants on Sunday night appeared poised to move on the provincial capital, as well as the centre of neighbouring Takhar province, after demoralised and poorly equipped Afghan troops either surrendered or retreated.

More than 300 Afghan military personnel crossed the border to escape the advance, Tajikistan’s State Committee for National Security said.

The loss of districts which were once a bastion of the Northern Alliance who fought the Taliban in the late 1990s is the latest reverse for an Afghan government which has appeared in disarray as American troops withdraw.

Afghanistan hardware: regional control

Government and anti-Taliban leaders were on Sunday night holding emergency talks after the inroads into one of the few regions then to hold out against the Taliban’s regime. The area has remained a rich recruiting ground for the Afghan army.

“Unfortunately, the majority of the districts were left to the Taliban without any fight,” Mohib-ul Rahman, a provincial council member, told AP. In the past three days, 10 districts fell to the Taliban, eight without a fight, he said.

The United Nations on Saturday told other aid agencies that it would start drawing down staff from Faizabad.

All major towns and cities remain under government control, but the speed of the collapse has alarmed Washington and its Nato allies, who fear it will become an unstoppable cascade of Taliban victories.

The Pentagon has said it will continue to give funding and advice to the Afghan forces once US troops leave, as well as fix its small fleet of planes and helicopters.

However, the swift Taliban advances have led US intelligence assessments to cut the government’s chances of survival, with analysts now predicting the government could fall within six to 12 months.