The shadow foreign secretary said Mr Assad, who has presided over a civil war which has killed more than 450,000 people and displaced 11 million, may not be “as overwhelmingly unpopular as the rebels told the West".

The Assad regime, which has been backed by Russia since 2015, has repeatedly been accused of using chemical weapons.

In an interview with the magazine Prospect, Ms Thornberry said: “There is an argument that if [President Assad] had been as overwhelmingly unpopular as the rebels told the West at the outset, then he wouldn’t be there.

“I think there has been a depth and a breadth of support for Assad that has been underestimated.”

Corbyn leads debate into Parliament vote on Syria strikes: 'The government is attempting to overturn a democratic advance'

Elsewhere in the interview, Ms Thornberry said foreign forces, including Britain, which has struck Isis targets as well as the Syrian regime's chemical weapons facilities, need to leave the country.

"They're not fighting for the sake of the Syrian people," she said.

She also suggested the UK should back peace talks overseen by Russia in Sochi. “I think we should be working with whatever works, for the sake of the Syrian kids. None of this is revolutionary,” she said.

Ms Thornberry refused to denounce Russia for vetoing 11 UN Security Council resolutions, saying: "People will always block resolutions. If you look at the number of resolutions America has blocked, I mean that’s the way of politics.”

Syria bombings: US, UK and French military air strikes wreckage Show all 13 1 /13 Syria bombings: US, UK and French military air strikes wreckage Syria bombings: US, UK and French military air strikes wreckage The wreckage of the Scientific Studies and Research Centre in the Barzeh district, north of Damascus, which was targeted by the US, UK and France air strikes. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier films the damage of the Syrian Scientific Research Center surrounded by papers and rubble. AP Syria bombings: US, UK and French military air strikes wreckage Firefighrers extinguish smoke that rises from the damage. The Pentagon says none of the missiles filed by the U.S. and its allies was deflected by Syrian air defenses, rebutting claims by the Russian and Syrian governments. AP Syria bombings: US, UK and French military air strikes wreckage The wreckage of part of the Scientific Studies and Research Centre compound . AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Part of a building collapsing, surrounded by the wreckage. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Further damaged to the Scientific Studies and Research Centre compound. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Lt. Gen. Kenneth McKenzie, the director of the Joint Staff at the Pentagon, also says there also is no indication that Russian air defense systems were employed early Saturday in Syria. AP Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier sprays water on the wreckage. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Syrian state news agency SANA reported several missiles hit a research centre in Barzeh, north of Damascus, "destroying a building that included scientific labs and a training centre". AFP/Getty Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier films the damage. AP Syria bombings: US, UK and French military air strikes wreckage Damage to the Scientific Research Center building that was hit by the strikes. EPA Syria bombings: US, UK and French military air strikes wreckage The Scientific Studies and Research Centre was one of the targeted buildings by the US, UK and France. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Further damage to the centre. EPA

Her comments drew the ire of Kristyan Benedict, the UK campaigns manager for Amnesty International.

“Maybe @EmilyThornberry believes elections in Syria are ‘free & fair’ and believes the ludicrous numbers Assad gets,” he tweeted.

“Maybe she should talk to Syrian human rights activists who’ve had to escape Syria because they dared to call for genuine elections & democracy.”