Labour leader Jeremy Corbyn called for debate on a ‘War Measures Act’ that would limit the government’s power to take military action without the approval of MPs.
And the Scottish First Minister Nicola Sturgeon said western allies had failed to exhaust all diplomatic means to pressure Bashar al-Assad’s regime to abandon its chemical weapons, and called for an emergency debate within 48 hours.
The government is likely to face calls for a full debate including a vote when Mrs May gives a statement to MPs on Monday on Saturday’s strikes, which saw British, French and US forces attack a handful of Syrian government military installations.
“The legal basis would have to be self-defence, or the authority of the UN security council,” the Labour leader said. “Humanitarian intervention is a legally debatable concept at the present time.”
“I think parliament should have a say in this, and the prime minister could quite easily have done that,” Mr Corbyn added.
“What we need in this country is something more robust, like a war powers act, so that governments do get held to account by parliament for what they do in our name.”
Speaking on Sky’s Sunday with Paterson programme, Ms Sturgeon said it was a “mistake” for the government not to consult parliament ahead of the military action.
“I don’t think we should see the involvement of the democratically elected parliament of the country in decisions like this as a sign of weakness,” the First Minister said.
“Indeed, I think it would be a sign of the strength of democracy. I think everybody would, and certainly I accept, that not all detail either of the intelligence or the military strategy should be shared, but asking parliament to sanction the broad objectives and the engagement of UK forces I think is something that should be done… we can’t turn the clock back, but I certainly think that should be done in future.”
Ms Sturgeon added: “In terms of the Prime Minister making a statement tomorrow, I would hope that becomes a full parliamentary debate.”
The SNP leader said she had “no difficulty whatsoever believing that Assad is capable” of using chemical weapons on his own people, but insisted that “the question here is what course of action is best place to alleviate humanitarian suffering that has been deepening in recent times”.
She added: “I don’t think there has been the concerted, patient effort by the international community. We’ve heard the UN envoy to Syria, Staffan de Mistura talk about this in recent weeks. There needs to be a real commitment behind those efforts.
“There’s also further action that can be taken in terms of the chemical weapons programme in Syria… i think there is more action that could be taken to disrupt its supply chains, to disrupt the transport to and from it, to apply sanctions to the key personnel in that research centre.
“There are very few individuals who are subject to UK sanctions. There are actually more who are subject to US sanctions.
“So when I hear the prime minister saying there is no practical alternative, I would question whether that is really the case.”