Britain has quietly halted its training of Syrian rebels, reflecting growing acknowledgment among key opposition backers that President Bashar al-Assad has victory in his sights.

Troops were sent last year to join US-led programmes in Turkey and Jordan, which aimed to train 5,000 vetted moderate opposition fighters a year for three years in the use of small arms, infantry tactics and satellite communication.

The Ministry of Defence confirmed to the Telegraph that in late June the last 20 soldiers returned home from the programme, which is ongoing.

The training was part of the Pentagon’s $500m (£390m) train-and-equip programme and was established in 2015 with the aim of pushing back Islamic State of Iraq and the Levant (Isil) in the east of the country and hold on to the territory to prevent its return.

Since an offensive launched earlier this summer by Syrian government troops and their allies around the US garrison at al-Tanf, south of Isil’s last major stronghold in Deir Ezzor, the rebels have wound down operations there.

The rest went on to fight the jihadists in Raqqa.

US and UK support was contingent on the fighters focusing their fire on Isil, not on Assad’s forces, so as not to be drawn into confrontation with the emboldened regime.

Assad’s maneuvering around Deir Ezzor, an oil-rich province which cuts the strategic Baghdad to Damascus highway and is highly prized by the government and its Iranian allies, shows his growing reach after a series of territorial gains.

Mozahem al-Saloum, a former spokesman for the rebel groups at al-Tanf who now runs an activist network that tracks developments in eastern Syria, said they lacked the men and weapons needed to lead the race to Deir Ezzor.

"Sadly, I think the regime will advance in the area before the opposition manages to announce any start of any battle from its side," he said.

An MoD spokesman told the Telegraph: “The UK training teams are currently held at readiness in the UK and will redeploy if required.”

While the British effort was relatively modest, the move is part of a broader trend of major powers scaling back both practical and political support for the opposition while at the same time softening their position on Assad.

Speaking last week, Boris Johnson, Foreign Secretary, said: “We used to say (Assad) has to go as a precondition. Now we are saying that he should go but as part of a transition. It is always open to him to stand in a democratic election.”

Diplomats have told the Telegraph that Mr Johnson, who had been among the most forceful in calling for the Syrian president’s removal, was confirming a shift in the UK’s position forced by events on the ground.

In July, President Donald Trump ended a parallel clandestine CIA programme which had for years been providing arms and supplies to a number of Syrian rebel groups fighting regime forces, saying that the US’s sole focus should now be Isil.

Saudi Arabia, which had been one of the staunchest backers of the rebel, told a delegation of opposition leaders at a summit in Riyadh late last month that they too would be disengaging.

Jordan more recently declared that “bilateral ties with Damascus are going in the right direction”, in one of the first signs of thawing relations.

A Kurdish fighter from the People's Protection Units (YPG) runs for a cover in Raqqa credit: REUTERS/Goran Tomasevic

With the help of Russian and Iranian forces, Assad has tightened his grip on power. His military has retaken strategic areas and major cities from the Syrian opposition, bussing surrendering moderate elements to the Islamist-controlled city of Idlib in the north in an attempt to present the West with a stark choice between him and extremists.

Assad this week boasted that the Syrian army’s victories have forced some foreign powers to change their stance on the conflict.

“Several countries have changed their positions after the victories of the Syrian army and its allies,” Assad said during a meeting with Iranian deputy foreign minister Hossein Jaberi Ansari.

“Most importantly, several countries have taken measures aiming to suppress the financing of terrorists remaining in Syria,” said the president, who uses the catch-all term to refer to his opponents.

The US, UK and its allies have all declared their only immediate aim now in Syria is to defeat Isil.

The US-led coalition has been concentrating its efforts on the Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters, battling Isil in Raqqa, the capital of the jihadists’ so-called caliphate.

A few dozen SAS soldiers are currently in the city, alongside hundreds of American special forces, advising the forces. RAF Typhoon and Tornado jets and Reaper drones are providing air cover.

After the fall of Raqqa and Deir Ezzor, however, it is unlikely what is left of the moderate opposition will have enough support to take on the regime.

Analysts say they are not surprised by Britain’s move, with some describing it as pragmatic.

“This looks like part of the trend where opposition-supporting outside powers are giving up on Syria, or to be more precise: redefining "success" solely in terms of getting Isil out of the cities and preparing to quickly withdraw once that is completed,” said Kyle Orton, a research fellow at the Henry Jackson Society think-tank.

Damaged building are pictured during the fighting with Islamic State's fighters in the old city of Raqqa, Syria, credit: Reuters

“It's a sort of bureaucratic rubber stamp to us accepting Assad remaining.”

Hamish de Bretton Gordon, a former British army officer who advises NGOs in Syria, told the Telegraph he believed the UK has had to accept the change of situation on the ground and stopped “banging its head against a brick wall”.

“We are now looking, strategically, elsewhere in Syria, which is understandable,” Mr de Bretton said. “But we must not abandon our responsibility to the fighters we trained, who will be looking for guarantees of protection in the future.”

He said Britain should now throw its support behind the stalled United Nations peace process in Geneva, where Assad could soon have a seat at the table.

Staffan de Mistura, UN special envoy to Syria, said on Friday that what we were seeing was “the beginning of the end of the war”, but stressed a military victory without a political settlement would only lead to an endless cycle of violence.

“If the international community helps both the opposition and the government by pushing [them] to accept a real negotiation,” he said, “then within a year it would be a possibility of having a truly credible election.”

Additional reporting by Luna Safwan in Beirut