Jeremy Corbyn will try to force a vote in Parliament that would make it difficult for Prime Ministers to take military action without the approval of MPs as he suggested Bashar al-Assad could be innocent of last week’s chemical weapons attack.
The Labour leader suggested on Sunday that all planned use of force should be signed off by the Commons as he announced plans for a “war powers act” which would ensure that all governments are accountable “for what they do in our name”.
Following raids on the Syrian regime's chemical weapons stockpiles on Saturday, Mr Corbyn questioned the legal basis for the mission, adding that Mrs May should have respected a convention supposedly laid down by the coalition government in 2011.
Labour sources indicated that Mr Corbyn will apply to the Speaker on Monday for an emergency debate under a Parliamentary mechanism called Standing Order No24. It allows MPs to call for a debate within 24 hours on matters of national importance.
The Prime Minister will also ask for an emergency debate, but while her bid will not include a request for a vote, Mr Corbyn is likely to ask for a vote which could include a call for Prime Ministers to consult Parliament in future.
Although such votes are not binding, any defeat for the Prime Minister would be humiliating and would make it politically more difficult for her to take military action in future. It is up to the Speaker to decide whether to allow either of the applications.
Mr Corbyn told the BBC: “There is precedent over previous interventions where parliament has had a vote, and I think 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.
"She could have recalled parliament last week - it is only the Prime Minister who can recall parliament - or she could have delayed until tomorrow when parliament returns.”
Despite receiving intelligence briefings on the chemical attack in Douma, Eastern Ghouta, Mr Corbyn again refused to blame the Assad regime, suggesting that “other parties” had access to chlorine gas and could not be ruled out.
His claims appeared to clash with Emmanuel Macron, the French President, who said that he had “proof” of Assad’s culpability, while a number of eyewitness accounts also place a regime helicopter in the area at the time of the gassing.
Mr Corbyn also questioned the legality of the strikes, suggesting that the Government's legal justification - humanitarian grounds - was not universally accepted by other countries.
“If we want to get the moral high ground, as a country with a history of international involvement, then we need to abide by international law,” he added. “I say to the Foreign Secretary, and I say to the Prime Minister, where is the legal basis for this?”
Mr Corbyn has also written to the Prime Minister asking that the advice provided by the Attorney General be published.
However, his decision to undermine the Government was heavily criticised on social media, with Labour MP John Woodcock describing Mr Corbyn’s stance as “deeply troubling”.
Brandon Lewis, the Conservative Party chairman, said that Mr Corbyn’s refusal to blame the Assad regime showed he was “more worried about upsetting Russia than about preventing use of chemical weapons”.
“Corbyn seems determined to obfuscate to avoid showing leadership on chemical weapons and UK defending itself or most vulnerable in the world,” he added.
The Labour leader also faced criticism when he refused to condemn “demented” claims made by the Russian foreign minister Sergei Lavrov, who has now suggested that both the Salisbury and Douma attacks were orchestrated by the UK.
When asked what he made of the comments, Mr Corbyn said: “I’m quite surprised. He’s either got to back it up or withdraw it.”
In the wake of the Salisbury attack, the evidence against Russia’s culpability has continued to grow, with newly declassified intelligence now showing that the Kremlin hacked Yulia Skripal’s emails for at least five years.
The intelligence, released by Sir Mark Sedwill, Theresa May’s National Security Adviser, also shows that Russian agents have tested the effectiveness of Novichok as a weapon for carrying out assassinations.
In a letter sent to Nato Secretary General Jens Stoltenberg , Sir Mark revealed that the agent was being smeared on door handles as part of a secret chemical weapons programme codenamed Foliant in which President Vladimir Putin was “closely involved”.
But when asked whether he was prepared to lay point the finger at the Kremlin, Mr Corbyn said that he would require “incontrovertible evidence” of Russia’s involvement.
Rounding on the Labour leader, Boris Johnson, the Foreign Secretary, said he was struck by his failure to dismiss the allegations with the “sort of vehemence and vigour you might have expected”.
“It is quite extraordinary in the view of the weight of evidence now...to continue to deny the likelihood of Russian involvement, of state-sponsored involvement,” Mr Johnson added.
“Quite extraordinary and a blindness to reality. I find it very, very perplexing. A defiant refusal to accept that the Kremlin is responsible.”
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