The coffin of John Stollery is carried from an RAF C-17, which repatriated the bodies of eight British nationals killed in the Tunisia terror attack, at RAF Brize Norton near Oxford in Britain July 1, 2015.[Photo/Agencies]

The question would need to be "properly and carefully deliberated" before Cameron decided whether or not to call a vote, his spokeswoman said.

"He thinks it would be better if there was consensus for such action," she added.

Cameron himself believed there was a legal case to take such action, she said, adding that Britain was already flying drones and planes over Syria to gather intelligence that could be used in any future strikes.

"We have assets that are flying over both Iraq and Syria," said the spokeswoman, saying Islamic State had lost 25 percent of its territory since the coalition began air strikes in Iraq.

She rejected the idea that such strikes in Syria could strengthen President Bashar al-Assad, who Cameron has in the past said is part of the problem not the solution.