The leader of Britain's opposition Labour Party, Jeremy Corbyn, said on Sunday that the legal basis used to support British airstrikes in Syria was debatable, adding that he would only support action backed by the United Nations Security Council.
"I say to the foreign secretary, I say to the prime minister, where is the legal basis for this?" Corbyn said.
"The legal basis ... would have to be self defence or the authority of the UN Security Council. The humanitarian intervention is a legally debatable concept at the present time," he said in an interview with the BBC.
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Asked by CNN on Sunday why parliament wasn't asked for its approval for Syria attacks, British Foreign Minister Boris Johnson says our main concern was the effectiveness and speed of the operation in Syria.
Britain will study the "options" with its allies if President Bashar al-Assad again uses chemical weapons against his people in Syria, but as yet there is nothing planned, Johnson also said on Sunday.
Johnson backed Prime Minister Theresa May's decision to join the United States and France to strike chemical weapons facilities in Syria on Saturday, saying it was the right thing to do to deter the further use of chemical weapons.
"There is no proposal on the table at the moment for further attacks because so far thank heavens the Assad regime have not been so foolish as to launch another chemical weapons attack," he told the BBC's Andrew Marr show.