The government is considering holding a vote to expand military action in Syria if the Conservatives win a big enough majority in the general election. Theresa May is believed to want Commons backing in order to have the freedom to join the US in airstrikes against the forces of Syrian president Bashar al-Assad in the event of another chemical attack on the rebels, according to a Whitehall source.

The UK is keen to line up fully alongside the US– the country is already engaged alongside its American counterparts in military action in both Syria and Iraq against Islamic State, but has not joined in the airstrikes against Assad’s forces.

The US president, Donald Trump, in one of his first interventions overseas, ordered a strike against a Syrian airbase on 4 April after an alleged use of chemical weapons against rebels at Khan Sheikhoun in Idlib province.

For the UK to mount similar punitive action against Syrian forces, the government would have to overturn a Common vote in 2013, when MPs, including Conservative rebels, voted against action against Assad after an earlier alleged chemical attack.

The foreign secretary, Boris Johnson, said last month that the UK might launch such airstrikes without parliamentary approval, but it is understood that the government would rather get parliament’s backing.

The Conservatives are turning the election into one not only about Brexit but also defence, claiming that the Labour leader Jeremy Corbyn would be weak on security issues. The UK’s Trident nuclear weapons system became an issue within days of the prime minister announcing the general election, and the Conservatives are also planning to push Corbyn over whether he would maintain existing levels of defence spending.

The Labour leader, in response to Johnson’s comments on Syria, told the BBC: “We don’t need unilateral action. We need to work through the UN but, above all, we need to bend ourselves totally to getting a political settlement in Syria.”

Conservatives, expressing confidence in securing a vote this time around, argue that the mood has changed since 2015 and that there is less tolerance towards chemical attacks. US and UK intelligence agencies, as well as military officials from both countries, appear confident that the Assad forces were behind the chemical attack based on aerial photographs of the craters, which they say shows the attack could only have been mounted from planes. The Syrian rebels have no planes.



Two Syrian generals alleged to have been involved in earlier attacks were also reported to have been spotted at the airbase hours before the attack. Samples from victims and soil were sent to the top-secret research facility at Porton Down near Salisbury for analysis. The samples were found to contain sarin, or something close to it.

According to US and UK surveillance, only Syrian planes were in the air at the time. Russian planes were not spotted until four hours later, which apparently rules out their involvement.

If there was to be another chemical attack, the US could mount another, more destructive attack on the Syrian airbase and planes elsewhere in the country.

After the 2013 vote, partly a result of a backlash against UK involvement in the 2003 Iraq invasion, MPs backed action against Isis in December 2015.

David Davis, the Brexit secretary, Julian Lewis, chair of the Commons defence committee, and Crispin Blunt, chair of the Commons foreign affairs committee, were among the 30 Conservative MPs to rebel against David Cameron’s motion at the time.

Johnson last week said that he and the prime minister agreed that in the event of another chemical attack, it would be difficult for the UK to ignore a request from the US to join further airstrikes. “If the United States has a proposal to have some sort of action in response to a chemical weapons attack, and if they come to us and ask for our support, whether it is with submarine cruise missiles in the Med or whatever it happens to be, in my view, and I know this is also the view of the prime minister, it would be very difficult for us to say no,” he told the BBC.

May, asked about Johnson’s remarks, did not deny that UK military intervention was an option, saying only that it was hypothetical and that there were no proposed strikes on the table.

In response to Johnson’s remarks, the Liberal Democrat leader, Tim Farron, said: “Critically, the government must seek the consent of parliament. Gone are the days where a prime minister can take us to war without democratic backing unless in exceptional cases of national security. May would be wise not to use the cover of an election to push this through.”