British MPs from across the political spectrum called for a free vote on the looming intervention with Syria when Parliament is recalled.
As David Cameron prepared to meet with intelligence chiefs and senior ministers at the National Security Council, Labour leader Ed Miliband indicated that his party would consider supporting international action, "but only on the basis that it was legal, that it was specifically limited to deterring the future use of chemical weapons, and that any actions contemplated had clear and achievable goals".
And Deputy Prime Minister Nick Clegg said Britain would "set a very dangerous precedent indeed" if it stood back and failed to act.
He was swift to give reassurances that any action would not be an "open-ended military" intervention in Syria with a goal of toppling the Assad regime, but rather legal, proportionate and specific measures to send out a "clear signal" that use of chemical weapons is intolerable.
But other politicians have expressed grave reservations, many taking to Twitter to voice their concerns.
Labour MP Diane Abbott said she may be forced to resign from the frontbench if party leader Miliband supported intervention.
Abbott, the shadow health minister, told The Guardian she was opposed to British involvement based on the current available evidence.
"I voted against the Iraq War. At the moment, I can't see anything that would make me vote for intervention in Syria," she said.
"Essentially it's a civil war. What Libya and Egypt have taught us is that these situations in the Middle East are complex. It's not good guys in white hats and bad guys in black hats."
Asked if she would resign her position if Labour supported military action, she said: "It would put me in a very difficult position."
Abbott spoke of her fears of becoming involved in another conflict without the backing of the UN.
She said: "If you make a military intervention in Syria, you have to take ownership of the situation. I think it would be wrong... I wouldn't support anything that wasn't legal and the only legal way would be UN backing."
The NSC is expected to discuss the intelligence gathered by United Nations inspectors from their visit to Mouadamiya, where last week's suspected chemical weapons attack is believed to have taken place.
Cameron said before the meeting that any intervention in Syria would not be about the conflict itself, but preventing the use of chemical weapons by any regime.
Decisions about British involvement have not been taken, he said yesterday, adding Parliament was the "right place to set out all of the arguments".
He said action must be "proportionate, have to be legal, would have to specifically be about deterring the use of chemical weapons".
Cameron, whose request to recall Parliament early was granted by the Speaker, said tomorrow's debate would ensure "proper" scrutiny and allow the Government to listen to MPs.
"Obviously this is a developing situation, as I say, decisions have not been taken, but we shouldn't stand by when we see this massive use of chemical weapons and appalling levels of suffering," he said.
"I think in Parliament is the right place to set out all of the arguments, all of the questions.
"But I would say this to people - there is never 100% certainty, there is never one piece or several pieces of intelligence that give you absolute certainty.
"But what we know is this regime has huge stocks of chemical weapons. We know they have used them on at least 10 occasions prior to this last widescale use.
"We know they have both the motive and the opportunity whereas the opposition does not have those things and the opposition's chance of having used chemical weapons in our view is vanishingly small."
The NSC includes Defence Secretary Philip Hammond, Home Secretary Theresa May and Deputy Prime Minister Nick Clegg among its members.
Foreign Secretary William Hague has warned in an op-ed for the Telegraph that Britain cannot allow the use of chemical weapons to go unchallenged, saying Britain "cannot allow diplomatic paralysis to be a shield" for Assad.
US President Barack Obama has yet to take a decision on whether to go ahead with a military strike against Syria, the White House has said.
State Department deputy spokeswoman Marie Harf said there was no doubt about who was responsible for last week's chemical weapons attack.
She told BBC Newsnight: "We are making our own decisions, on our own timeline.
"Right now we have determined that the Assad regime did in fact use chemical weapons against his own people; our intelligence assessment will be shared with the public this week at some point.
"But let there be no question about who is responsible here and the fact that President Obama will be making decisions about our own response, on our own timeline."
Meanwhile, the former British ambassador to Washington Sir Christopher Meyer said a military strike against Syria would be "the decision from hell".
Speaking in a pre-recorded interview on Newsnight he said: "It cannot be in the British national interest to see Assad disintegrate under the pressure of cruise missile attacks, and whatever else may be done, such that his stocks of chemical weapons fall out of his control into the hands of the extremist Jihadists among the rebels.
"This is why this decision on what to do next is truly the decision from hell."
The British public, too, has yet to be persuaded. A YouGov survey for The Sun revealed that nearly three-quarters of people oppose the deployment of British troops to Syria, and a majority of 3-1 believe the Government should be bound by Parliament's vote tomorrow.Suggest a correction