Labour leader Jeremy Corbyn has accused the Government of 'waiting for instructions' from US president Donald Trump on what to do over Syria.
He said: "Further UK military intervention in Syria's appalling multi-sided war risks escalating an already devastating conflict.
"The Government appears to be waiting for instructions from President Donald Trump on how to proceed. But the US administration is giving alarmingly contradictory signals.
"Even US defence secretary James Mattis has said we 'don't have evidence' and warned further military action could 'escalate out of control'."
Mr Corbyn added: "Ministers should take their proposals, such as they are, to parliament. And Britain should press for an independent UN-led investigation of last weekend's horrific chemical weapons attack so that those responsible can be held to account.
"Rather than further military action, what is urgently needed is a coordinated international drive to achieve a ceasefire and a negotiated settlement under UN auspices. The humanitarian priority must be to halt the killing on all sides.
"The need to restart genuine negotiations for peace and an inclusive political settlement of the Syrian conflict, including the withdrawal of all foreign forces, could not be more urgent. We must do everything we can, no matter how challenging, to bring that about."
Labour's shadow home secretary Diane Abbott told BBC Radio 4's Today programme: "It would be outrageous for the Government not to bring military action in Syria to Parliament, for Parliament to have a vote. And the Tories used to think like that.
"The reason they are not doing it is they are frightened they will lose the vote."
Pressed on whether there were any circumstances in which Labour would back military action in Syria, Ms Abbott said: "What we are interested in is an end to the violence and we don't believe that further bombing, in this situation, will bring an end to the violence."
Asked if Russia or the US posed the greatest threat to world peace, Ms Abbott said: "It's clear that at this point Russia, its role in Syria, what we believe, beyond reasonable doubt, is its role in the poison gas attacks in Salisbury, is a greater threat to world peace than the United States."