A row has erupted over No 10's claim Labour is giving "succour" to Syria's regime by not backing the prime minister over military action there.
Labour is demanding an apology for what it describes as "infantile" comments.
Downing Street is reported to be furious that Labour leader Ed Miliband has not backed David Cameron's motion paving the way for military strikes.
The row erupted as the Commons vote rejecting the government's motion on intervention in Syria approached.
Deputy Prime Minister Nick Clegg distanced himself from Downing Street's claims about Mr Miliband - and said the prime minister "agreed with me".
But Mr Clegg told MPs the government's motion was "very tightly defined" and the intention was not to "topple a dictator" .
"The sole aim is the relief of humanitarian suffering by targeting and disrupting the further use of chemical weapons," he told MPs.
He urged MPs not to let their scepticism about attacks, in the wake of the Iraq war, get in the way of doing the right thing and he assured them that there would be separate debate and vote before any military action is launched.
Mr Clegg's refusal to criticise Mr Miliband appeared to put him at odds with Defence Secretary Philip Hammond, who earlier stood by claims the Labour leader was giving "succour" to Assad.
"Anything that stops us from giving a clear united view of the British Parliament tonight will give some succour to the regime," he told Channel 4 News.
"We deliberately structured our motion to take account of the concerns the Leader of the Opposition had expressed directly to us.
"But he has still chosen to table an amendment and ensure that we don't have a clear, united and unified opinion from the British Parliament."
MPs had been recalled from their summer break early to vote on whether the UK should join in US-led strikes on Syria, if they go ahead.
Hundreds are reported to have died in the attack near Damascus on 21 August. The Syrian regime denies any involvement, blaming opposition forces.
But the prime minister was forced to water down the government's motion after Labour refused to back it and a second vote will now be needed to authorise military strikes.
Mr Miliband will still order his MPs to vote against the government, saying he needs to see more evidence that Syrian President Bashar al-Assad was behind the attack.
The majority of Labour and Tory MPs to take part in the marathon emergency debate have expressed doubts about the wisdom of attacking the Assad regime.
And Labour MP Jim Fitzpatrick quit his frontbench role as shadow transport spokesman.
Earlier in the Commons, Mr Fitzpatrick said he would vote against both the government and Labour's amendment as he opposes military intervention of any kind.
This is despite Mr Cameron's insistence that action would be justified to prevent further "war crimes" and would be in line with international law, even if it was opposed at the UN by Russia and China.
BBC Political Editor Nick Robinson said he believed the government would still win Thursday night's vote - partly because Conservative MPs would be unwilling to hand Labour a victory.
But a bitter war of words has broken out between Downing Street and the Labour leader, who they accuse of "playing politics" with the Syria issue.
According to The Times newspaper, a government source used a string of expletives to describe Mr Miliband's attitude and said: "The French hate him now and he's got no chance of building an alliance with the US Democratic Party."
Downing Street director of communications Craig Oliver is reported to have accused Mr Miliband of giving "succour to Assad".
Labour frontbencher Michael Dugher has written to Cabinet Secretary Jeremy Heywood to complain about what he called Mr Oliver's "infantile and irresponsible" comments, which he claimed "demeans the office of prime minister".
Setting out the government's case at the beginning of the special Commons debate, Mr Cameron described last week's attack on the outskirts of Damascus as "one of the most abhorrent uses of chemical weapons in a century, slaughtering innocent men, women and children".
He said: "Interfering in another country's affairs should not be undertaken except in the most exceptional circumstances. It must be a humanitarian catastrophe and it must be a last resort.
"But by any standards, this is a humanitarian catastrophe, and if there are no consequences for it there is nothing to stop Assad and other dictators from using these weapons again and again."
He said it was "not about taking sides in the conflict" or "regime change" but responding to a "war crime".
In a swipe at his Labour predecessor, Tony Blair, Mr Cameron said, "The well of public opinion has been well and truly poisoned by the Iraq episode."
But he insisted the current crisis was not like Iraq and MPs would "decide which next steps" the UK would take.
Mr Miliband told MPs he was not against military intervention. But he said Britain had to be "clear-eyed" about the possible consequences of such action - including deepening Britain's involvement in Syria's bitter civil war.
He said Britain should not make the decision based on an "artificial timetable or political timetable set elsewhere", but should instead follow Labour's "sequential road map" - which includes gathering "compelling evidence" that President Assad's regime was to blame for last week's attack.
"I'm not with those who rule out action - the horrific events unfolding in Syria do ask us to consider the options available," said Mr Miliband.
"But we owe it to the Syrian people, to our own country and to the future security of our world to scrutinise any plans on the basis of the consequences they have."
Downing Street has released a statement, based on legal advice by Attorney General Dominic Grieve, that states limited military strikes to deter future chemical weapons attacks would be in line with international law.
An assessment published by the Joint Intelligence Committee also argued it was "not possible for the opposition to have carried out a chemical weapons attack on this scale".
However, Labour MPs will vote there must be "compelling evidence" that the Syrian regime was responsible for the use of chemical weapons. Plaid Cymru and the SNP will also back the Labour amendment.
Conservative MP Sir Malcolm Rifkind, chairman of the parliamentary Intelligence and Security Committee, was one of the relatively few backbench voices to back military action.
He told MPs failing to act would give the Syrian regime in Damascus a rationale for using chemical weapons in the future.
Another former foreign secretary, Conservative peer Lord Hurd said: "I cannot for the life of me see how dropping some bombs or firing some missiles in the general direction of Syria" will lessen the "suffering of Syrian people".
"I think it's likely to increase and expand the civil war in Syria."
Labour MP Jack Straw, who was foreign secretary when the UK joined the US in attacking Iraq in 2003, said that conflict had raised the bar on the quality of intelligence needed for military intervention and he was not convinced there was enough evidence yet to justify action in Syria.
"We all know - I have the scars about this - how easy it is to get into military action and how difficult it is to get out of it," he told MPs.
Syria has accused the West of "inventing" excuses to launch a strike and says a UK strike would be an "aggressive and unprovoked act of war".