IT would be a “scandal” if Theresa May did not grant a full parliamentary debate and vote on UK military involvement in Syria, the SNP has insisted.

Pressure is building on the Prime Minister to ensure MPs have their say on the use of British military assistance or hardware in any US-led strike on the Assad regime over the suspected poison gas attack on Douma last weekend.

Mrs May appeared to pave the way for the UK’s involvement after stressing how "all the indications” were that President Bashir al-Assad’s Government was indeed responsible for the attack on Douma and she declared: “The continued use of chemical weapons cannot go unchallenged.”

But in a letter to her, Stewart McDonald, the SNP’s defence spokesman, urged extreme caution, stressing how his party could not support a strategy that did not bring the war to an end but only escalated the violence.

While condemning in the strongest terms the use of chemical weapons, the Glasgow MP said he was “not convinced” retaliatory airstrikes being proposed by Donald Trump was the way forward.

“The UK is only actively engaged in Syria because the UK Government carried a motion on December 2 2015 in which it asked Parliament to approve the 'exclusive' targeting of Daesh in that country. If the UK Government wishes to expand its involvement, then it would be a scandal if it did so without a full debate and vote in the House of Commons,” said Mr McDonald.

Jeremy Corbyn also insisted there would have to be a Commons vote on any plan for the use of the UK’s military.

During a campaign visit to Plymouth, the Labour leader said: “Parliament should always be given a say on military action. That’s a case that I’ve made going back many, many years in Parliament.

"Obviously, the situation is very serious. Obviously, there has to be, now, a demand for a political process to end the war in Syria. We cannot risk an escalation even further than it’s gone already," he added.

In 2013, David Cameron failed in his bid to get parliamentary backing for military action against the Assad regime following a chemical attack on the Syrian people.