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More than 80 Conservative MPs have written to David Cameron about their "very real concerns" about the idea of Britain helping to arm Syrian rebels.

They fear arms could fall into the wrong hands and the conflict could be prolonged.

They have demanded a debate and vote in the House of Commons before any decision is taken. The Leader of the Commons Andrew Lansley has offered them a debate and vote but no guarantee of when it might take place.

Labour and some Lib Dems have voiced their concerns too. And while reports of Cabinet divisions have been exaggerated, there is certainly unease in the upper reaches of government.

Voices within the Foreign Office fear the memory of the Libyan intervention is stalking the corridors of No 10. They dread a rush of blood to the head in response to some bloody incident or massacre in the future.

Some Conservative MPs are more hopeful. They claim that the prime minister was taken aback by their opposition when he talked about Syria in the Commons on Monday.

They contrast what they describe as his gung-ho tone on Monday with his rather more cautious approach at PMQs on Wednesday, where he twice insisted that no decision had been taken. These MPs also predict that the PM would lose any vote on arming Syrian rebels.

The official position, as articulated by the prime minister's official spokesman this week, is that there is "complete agreement" within government over the approach, namely that the EU arms embargo was lifted "to crank up the pressure" on the Assad regime to encourage them to come to the negotiating table in Geneva next month.

Lifting the embargo simply "gives us flexibility down the line", he said.

But I have been told by a source familiar with the prime minister's thinking that he is only "weeks away" from making a decision about whether or not to arm Syrian rebels.

There are some around the prime minister who are arguing strongly for a decision to intervene.

They say that Assad is convinced he can win a military victory. The only way to dissuade him of that, they say, is by arming the rebels. Otherwise his regime will never come to the negotiating table.

"Those opposed (to arming the rebels) are saying Assad should win," said one source.

"Is that really the policy that the PM should follow? If the balance on the ground doesn't change, if it continues on indefinitely, the prediction of the UN is that by the end of the year half the Syrian population - that's about ten million people - will be displaced in some shape or form.

"This is a humanitarian crisis of an enormous scale. To do nothing is not a very courageous position."

The source also noted that the longer the West waits to arm the rebels, the stronger the more extremist wings of the Syrian rebels become.

So this debate is rather more finely balanced than some have made out. And the stakes are extremely high.