Mr Johnson said Russia could win “the acclaim of the world” by halting bombing and bringing Mr Assad to the negotiating table.

But Lord Richards said if the West was interested mainly in the humanitarian plight of civilians, then “I believe there is a strong case for allowing Assad to get in there and take the city back.

“The opposition groups – many of whom are not friends of ours, they’re extremists – are now intermingled with the original good opposition groups, are fighting from amongst the people.

“The only quick way of solving it is to allow Assad to win. There’s no way the opposition groups are going to win.”

He said Hillary Clinton should not raise the prospect of no-fly zones over Syria to stop air strikes, “unless she’s prepared to do this properly and go to war with Russia”.

He said: “Unless she’s prepared to do this properly and go to war with Russia, she shouldn’t talk about no-fly zones and nor should we. We would have to shoot down Russian aircraft in order to impose it. Do we really want to go to a shooting war over Aleppo?

“We want the humanitarian horror of Aleppo to come to a rapid halt. The best and quickest way of doing that is to encourage the opposition groups to leave.