Jeremy Corbyn has attacked the Government for launching "legally questionable" air strikes in Syria.
Prime Minister Theresa May called the Labour leader late on Friday night to discuss the strikes and gave him an updated security briefing.
But the talks failed to persuade Mr Corbyn that launching cruise missiles in response to the chemical weapons attack in Douma was the right course of action.
Despite Russia repeatedly blocking action against Syria through the United Nations, Mr Corbyn called for Britain to go back to the group of world powers to secure a resolution that has Moscow's backing.
He was written to the Prime Minister to say he believes the strikes were "legally questionable" and said MPs should have been given a vote.
Labour's leader in Wales, First Minister Carwyn Jones, struck a different tone in a statement issued following the strikes.
He said: "I spoke with the PM late last night about the action in Syria.
"I offered my support to any intervention that could prevent a further atrocity, but it is vital that any action forms part of a wider long-term plan for the region."
Downing Street is expected to release a summary of the legal advice given to Mrs May ahead of the decision to join the co-ordinated assault with the United States and France.
Mr Corbyn insisted, however, that the full document must be released as he called for "renewed diplomatic efforts" the bring the conflict in Syria to an end.
In the letter to Mrs May, Mr Corbyn wrote: "I believe that Parliament should have been consulted and voted on the matter. The UK Prime Minister is accountable to Parliament, not to the whims of a US pPresident.
"I believe the action was legally questionable, and this morning the UN Secretary-General has said as much, reiterating that all countries must act in line with the UN Charter.
"You assured me that the Attorney General had given clear legal advice approving the action. I would therefore be grateful if you would publish this advice in full today."
Russia, the Syrian regime's most powerful ally, used its veto at the United Nations to prevent an investigation into the attack on the rebel-held town of Douma one week ago. It was the latest in a series of occasions when Moscow has blocked action against Syria during the seven-year civil war.
The Labour leader, however, said the UK must go to the UN to secure a new resolution that has the backing of Russia and the United States along with Iran, Saudi Arabia, Israel, Turkey.
He said an agreement secured in 2013 between then US secretary of state John Kerry and Russian foreign minister Sergey Lavrov on the destruction of chemical weapons in Syria showed an agreement could be reached.
"There is precedent that this process can work, and surely it would have been better to do that than start bombing - and goodness knows what the consequences of the bombing could be," he said.