Boris Johnson has insisted Russia could still be hit with tough sanctions over its support for Syria after he was attacked by members of his own Government for failing to build an international coalition to take on Vladimir Putin.
The Foreign Secretary left a G7 summit of foreign ministers without securing backing for his plans for sanctions after Italy, France and Germany publicly rebuffed him, saying Mr Putin must not be “pushed into a corner”.
A communiqué issued by the G7 made no mention of sanctions but included a significant new agreement that Bashar al-Assad cannot stay on as Syrian president.
One Government source told The Telegraph that the failure to secure an agreement on sanctions - which came on top of Mr Johnson cancelling a trip to Moscow - had been a "total let down".
But a spokesman for Mr Johnson said such a view was “completely short-sighted and wrong” because Russian and Syrian military figures would still face sanctions with the G7’s blessing if an independent investigation proves Assad was to blame for last week’s chemical attack and Mr Putin carried on backing him.
On Tuesday night the White House compared Assad to Adolf Hitler in his use of chemical weapons and asked the Kremlin “at what point do they realise they are getting on the wrong side of history in a really bad way?”
Rex Tillerson, the US Secretary of State, landed in Moscow to deliver the G7’s message that Mr Putin can either help solve the Syrian crisis or share Assad’s legacy of destruction.
Mr Tillerson said that Russia had either been "incompetent" or failed to take its obligations over chemical weapons seriously, but added that the distinction "doesn't much matter to the dead".
He added that "it is clear to all of us that the reign of the Assad family is coming to an end" and said of the recent chemical attack: "We cannot let this happen again".
Mr Johnson faced widespread criticism for failing to secure the G7’s support for sanctions, and a Government source told the Telegraph his credibility had been damaged as a result.
The source said: "He is now persona non grata with Russia, they won't take him seriously. He has blown his bridges with them by cancelling [a visit] for the third time.
"We have no credibility with the Russians and no additional credibility on the World stage following the G7. This has been a very negative experience."
The source claimed that there are increasing tensions between Mr Johnson and Downing Street over his handling of the crisis. "It's a very difficult moment for him, it was a play to increase his influence but it has backfired," a source said.
Downing Street, however, insisted that Mr Johnson had Mrs May's full support.
A Number 10 source said: “We are in a stronger position now than we were at the weekend because there is now agreement that Assad must go.”
Mr Johnson’s spokesman also insisted that “everyone around the table” at the G7 had agreed that “sanctions should and would be imposed on those involved in the chemical weapons attack once an investigation happens”.
President Putin ramped up tensions ahead of Mr Tillerson’s visit by claiming the chemical weapons in Syria were fake and compared them to the non-existent weapons of mass destruction in Iraq.
He said: “It reminds me of the events in 2003 when US envoys to the security council were demonstrating what they said were chemical weapons found in Iraq. We have seen it all already.”
He said Russia had information that the US is planning to launch further attacks in Syria. "We have information that a similar provocation is being prepared," he said.
Mr Johnson yesterday downplayed suggestions that Russia knew of the chemical attacks in advance, just days after Sir Michael Fallon, the Defence Secretary, said Mr Putin was responsible for the attacks "by proxy".
He said: “Did they know that Assad was going to unleash chemical weapons? We have no evidence for that, we don’t know whether the Russians were involved at all.”
Angelino Alfano, the Italian foreign minister and summit host, told the closing news conference: “We must have a dialogue with Russia and we must not push Russia into a corner. There is no consensus on additional new sanctions as an efficient instrument to deliver the goal we are aiming for.”
Jean-Marc Ayrault, the French foreign minister, said: “The question wasn’t mentioned by anyone except Boris Johnson but we didn’t talk about it any further."
Mr Johnson insisted that the option of sanctions remained on the table and said there would be an investigation by the Organisation for the Prohibition of Chemical Weapons.
“After that, if of course we can find people, whether they are Syrians or whether they are Russians associated with the Syrian military operation, it is in my view wholly appropriate that they should face economic sanctions or sanctions of some other kind,” he said.