Boris Johnson is a “liability” who is undermining Britain’s credibility abroad, Labour has said.

Shadow chancellor John McDonnell warned that the Foreign Secretary’s performance over Russia and Syria – and failed attempt to push for new sanctions – was reflective of “poor judgment”.

“The idea of going along to the G7 to try and put together a coalition around sanctions was never going to succeed. It was a bad policy decision by Mr Johnson. It should never have happened,” Mr McDonnell told Sky News.

“He should have gone to Moscow. When he went to Moscow he should have been clear with the Russians just how much we condemn what Assad has done, and what they’ve done in support of Assad, but try to get them back to the negotiating table.

“I’m hoping the initiative now by the Trump regime and others is going to enable that to happen. It’s a pity Mr Johnson made such a poor judgment of this matter.”

Mr Johnson was last week accused by domestic politicians and the Kremlin of being a “poodle” to the US after he cancelled his own visit to Russia to make way for US secretary of state Rex Tillerson.

The Foreign Secretary was also humiliated at a G7 meeting in Italy after he publicly pushed for sanctions on Syria and Russia but was rebuffed by his European allies.

John McDonnell criticised the Foreign Secretary (Getty)

Tory MP Alistair Burt, who was formerly a junior foreign office minister, however told BBC Radio 4’s Today programme the issue was “not about Boris Johnson”.

“What is most important here is that we try to find a way to end this conflict in which one out of two Syrians have either died or been displaced. It is going to be very difficult,” he said.

In pictures: US missile strike against Syria Show all 7 1 /7 In pictures: US missile strike against Syria In pictures: US missile strike against Syria The guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea AP In pictures: US missile strike against Syria The United States military launched at least 50 tomahawk cruise missiles at al-Shayrat military airfield near Homs, Syria, in response to the Syrian military's alleged use of chemical weapons in an airstrike in a rebel held area in Idlib province EPA In pictures: US missile strike against Syria Shayrat airfield in Syria Getty Images In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria President Donald Trump speaks at Mar-a-Lago in Palm Beach, Fla., after the US fired a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians AP In pictures: US missile strike against Syria Syria's President Bashar al-Assad Reuters

“It does involve some difficult and distasteful calculations, perhaps with Russia and Iran, but the main focus should not be on an intergovernmental spat about whether or not Boris Johnson has been humiliated.”