MPs have urged Theresa May not to commit to a military confrontation in Syria in response to the use of chemical weapons without a vote in parliament, while a former Labour frontbencher has suggested Ed Miliband was wrong to oppose action in 2013.

David Cameron lost a Commons vote in 2013 over proposed airstrikes to punish Assad’s use of chemical weapons, after the then Labour leader Miliband and Conservative rebels opposed the plan. May, who has a much slimmer majority in parliament than Cameron had, could risk her leadership being fatally undermined by a similar result.

The former shadow foreign minister Emma Reynolds said she now believed Labour had been wrong to oppose action in 2013, although she said parliament still needed to give its approval this time.

“In hindsight I think we were wrong to vote against reacting to the use of chemical weapons, it set a dangerous precedent, and [Barack] Obama saying it was a red line that couldn’t be crossed and then not taking action was the wrong thing to do. We cannot normalise the use of chemical weapons,” she told BBC Radio 4’s Today programme.

Jeremy Corbyn is almost certain to oppose further action in Syria and has said military intervention would require UN approval, currently blocked by Russia.

Emily Thornberry, the shadow foreign secretary, has previously said the government needs a new mandate to extend its bombing campaign in Syria, given that a previous vote only covered strikes against Islamic State, which is near defeat.

Conservative MPs, including those who voted against airstrikes in 2013, have also expressed caution. John Baron said there was a risk of escalation, especially with Russia.

“Parliament has every right to ask questions particularly given our past record of previous intervention across the region,” he said. “If you haven’t got clarity with regard to our objectives, you end up being led by events. Our track record of understanding events on the ground and the consequences of those actions has been poor.”

Julian Lewis, chair of the Commons defence committee, who did not back action against Assad in 2013, said MPs needed to vote on any escalation. “When we are contemplating military intervention in other people’s conflicts, parliament ought to be consulted,” he said.

The former attorney general Dominic Grieve said May could take some limited action without MPs’ approval, but a wider campaign in Syria would need the backing of parliament.

Some Tory MPs who favour action have argued May would not require a Commons vote or could hold a vote retrospectively when parliament returns from recess next week.

Tom Tugendhat, the chair of the foreign affairs select committee, said: “Striking Syria’s stockpiles of chemical weapons would degrade their ability to commit further war crimes and could be done together with allies. It would not require a vote in parliament.”

Nick Boles, a former minister, said May would have his “strong support” if she decided to join the US and France in taking action against the Syrian regime. “Parliament should have a chance to debate the crisis early next week, but this should not become an excuse for inaction or delay,” he said.

Gen Richard Barrons, who led the UK’s joint forces command until 2016, said on Wednesday he thought UK action alongside France and the US was the likely outcome. He said he hoped the response “extends beyond the tokenism of previous attempts which have been an act of event management and limited by scale and by time”.

May, Donald Trump and Emmanuel Macron held consultations by phone on Tuesday and said afterwards they had “agreed that the international community needed to respond to uphold the worldwide prohibition on the use of chemical weapons”.

Russia’s ambassador to Lebanon, Alexander Zasypkin, said on Wednesday that any US missiles fired at Syria would be shot down and the launch sites would be targeted.

Barrons said the ambassador should think carefully about his words: “He is saying … not only are they going to shoot down the missiles in flight … but by saying launch sites, he is saying they are going to try and sink ships, sink submarines and shoot aircraft out of the sky. That’s war.”