The leader of Britain's opposition Labour Party, Jeremy Corbyn, said on Sunday that the legal basis used to support British air strikes 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.