A former head of MI6 has hit out at Donald Trump for using Twitter to conduct critical diplomatic communications with Russia ahead of a potential conflict in Syria.

Sir John Sawers suggested Mr Trump’s use of social media would not provide the clarity of message to Moscow needed, as the world’s superpowers squared up following the alleged chemical weapons attack in the town of Douma.

The ex-British intelligence chief’s call for better lines of communication to the Kremlin came after the President took to Twitter on Wednesday, to tell Russia to “get ready” for missiles to hit Syrian Government targets.

UK and US military forces are reported to have moved into position for strikes on Syrian regime infrastructure, with Theresa May due to hold a ‘war Cabinet’ meeting later on Thursday afternoon.

Speaking to BBC Radio 4’s Today programme, Sir John said the West was back to a level of danger in relations with Russia not seen since the Cold War, and that Western leaders now had to be perfectly clear in communications with Moscow.

He said: “It is one of the weaknesses. I don’t put this down just to the current US administration.

“I think it goes back to the Obama administration, that failed to develop and invest in lines of communication at ambassadorial, or ministerial or head of state level, where although there are serious disagreements between Russia and the United States, there is at least clarity, so the scope for misunderstanding was minimised.”

US v Russia on Syria: The story so far

He went on: “We…need very clear commutation – not by tweet and I don’t approve of what President Trump is doing – but his instincts on this issue, that you do need to respond is basically right.

“The method for communicating with Moscow is not right.”

With tension rising on Wednesday, Russia’s ambassador to Lebanon, Alexander Zasypkinhad, warned that his country would shoot down US rockets fired at Syrian targets, and would even fire on “the sources that launched the missiles” – raising the prospect of direct engagement between US and Russian forces.

In a tweet responding to the threat, President Trump said: “Russia vows to shoot down any and all missiles fired at Syria.

Donald Trump: 'nothing is off the table' for US response to alleged Syria chemical attack

“Get ready, Russia, because they will be coming, nice and new and ‘smart’. You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!”

Downing Street sources said the Cabinet meeting – unusual during a parliamentary break – represented the “next phase” of Ms May’s approach to the crisis, following her conversations with Mr Trump, French President Emmanuel Macron and national security officials.

With the callout notice for the cabinet meeting sent on Wednesday night, the Prime Minister appeared to harden her language on the Douma incident, in which reports suggest bombs filled with a chemical substance were dropped on the town leaving more than 40 people dead.

Ms May said: “All the indications are that the Syrian regime was responsible and we will be working with our closest allies on how we can ensure that those who are responsible are held to account and how we can prevent and deter the humanitarian catastrophe that comes from the use of chemical weapons in the future.

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“The continued use of chemical weapons cannot go unchallenged.”

She told reporters during a visit to Birmingham that she was “appalled” but “not surprised” by Russia’s decision to veto a draft resolution at the United Nations on Tuesday, which sought to create a new body to determine responsibility for the attack.

Indicating that she believes the diplomatic route may have been exhausted, she said: “There can be no role now for investigations by the United Nations.”