Legal advice sought by Labour frontbencher Tom Watson has suggested the UK-backed air strikes in Syria went against international law and may be illegal.
Dapo Akande, professor of public international law at Oxford University, claims that "neither the UN Charter nor customary international law" allows for military action on the basis of humanitarian intervention.
The UK is "one of very few states that advocates for such a legal principle but the vast majority of states have explicitly rejected it", Akande said.
The legal opinion, released by Labour's deputy leader this morning, also claims that "to accept the position advocated by the government would be to undermine the supremacy of the UN charter".
Even if humanitarian intervention were covered, these strikes would not appear to meet the tests of bringing "immediate and urgent relief" to civilians, and that the action was taken before inspectors from the Organisation for the Prohibition of Chemical Weapons were able to reach Douma.
Inspectors said this morning they were still being blocked from the area, however.
But the government has received widespread support from MPs, both Labour and Conservative, although the opposition front-bench and many others have been highly critical of the act, particularly as it came without a vote in Parliament. They have also been heavily criticised by large numbers of members of the public in the countries which carried out the attacks on Friday.
MPs will debate the air strikes after the Prime Minister gives a statement to the House of Commons, which is due to take place from 3:30pm today.