A Cabinet minister who failed to support the Government's motion on Syria did not cast her vote because the bell that alerts MPs of a division in the House of Commons was apparently not rung.
International Development Secretary Justine Greening and Foreign Office minister Mark Simmonds failed to vote to back the principle of military action in Syria.
It is understood Ms Greening and Mr Simmonds were in a room near the Commons chamber discussing a different foreign policy matter and were unaware that the vote was taking place.
Both MPs voted against the Labour amendment at 10pm, which called for "compelling evidence" the Assad regime was behind the chemical attack.
Mr Simmonds then apparently asked to speak to Ms Greening about a separate Foreign Office issue and the pair went to a meeting room, where they apparently did not hear the bell for the second vote.