“The Russian intervention completely put the Syrian army back on its feet. I asked him why [Putin] downscaled it,” Mr Davis said. “[Assad] said: ‘Because Russia was being criticised for stalling the [peace] talks,’ taking away the incentive for him to negotiate it.

“But then there was a line that came out of it, almost as a throwaway line, he said: ‘Putin said, we will not let you lose.’ For me, that was in some ways the most important phrase of the entire visit.”

Mr Davis, a former minister and one-time Conservative leadership hopeful, said that an assurance of Russian support, if it had been given, meant there could only be a regime victory, or a negotiated settlement in Syria.