Theresa May has defended Boris Johnson’s handling of the Syria crisis, saying he had done an “excellent job” despite criticism of his failure to win backing for further sanctions on Russia.

The prime minister said Johnson had taken a very clear message to the G7 and managed to forge an international consensus on the need for Syria to have a future without President Bashar al-Assad, who is backed by the Russian regime.

Johnson has been accused of humiliating failure by Labour and the Liberal Democrats for misjudging the international mood when he tried to get backing from Italy, France and Germany for a tougher approach towards Russia. He was also criticised for cancelling a trip to Moscow for talks with his counterpart after the chemical weapons attack in Khan Sheikhun.

However, May strongly backed the foreign secretary during a visit to the Royal Military Academy Sandhurst, where she echoed Johnson’s argument that Russia is “on the wrong side of this argument”.

Assad has dismissed the alleged chemical weapons attack in Khan Sheikhun as a “fabrication” to justify US missile strikes against an airbase in his country. Russia also argues there is no evidence Assad was involved.

But May said British scientists at the Porton Down research facility were clear that the sarin nerve agent or a “sarin-like substance” was used in the 4 April attack and it was highly likely Assad’s regime was responsible.

“Boris Johnson did an excellent job in bringing together the G7, bringing together international opinion and enabling [US secretary of state] Rex Tillerson from the United States to go to Moscow with a very clear message – a consensus from the international community,” she said.

“Russia is on the wrong side of this argument but we are willing to work with Russia to bring an end to the conflict in Syria, to bring about a political solution in Syria, but that political solution has to be without Assad.

“That was the clear message the foreign secretary took to the G7 and the G7 gave to secretary Tillerson.”

