Defence Secretary Philip Hammond accused Ed Miliband of giving "succour" to the Assad regime as the Westminster battle over potential military action in Syria became increasingly bitter.
Mr Hammond said Labour's decision to oppose a Government bid to secure MPs' support in principle for an armed response to chemical weapons use could encourage the Syrian government.
As MPs, recalled from the summer break to debate the crisis, prepared to vote in the Commons, anger in Government ranks over Mr Miliband's stance developed into a heated row.
Labour lodged a complaint about the Prime Minister's top spin doctor after he accused Mr Miliband of potentially "giving succour" to Assad by delaying a decision on UK involvement in a military response.
It sent a letter to Cabinet Secretary Sir Jeremy Heywood condemning the "infantile and irresponsible" comments made by director of communications Craig Oliver. But asked about the remarks, the Defence Secretary repeated the allegation.
"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."
Mr Miliband's insistence that a decision on UK involvement should await the report of UN weapons inspectors examining the attack site in Damascus forced Mr Cameron to water down his motion.
Without Labour's support, the vocal scepticism of many Conservative MPs meant the Prime Minister was faced with a humiliating Commons defeat, unprecedented in the 20th century. He accepted the need for a second vote to authorise any involvement. But the concessions did not go far enough for Labour leader Ed Miliband, who tabled an alternative motion demanding "compelling evidence" that the Assad regime was responsible.
Making the case for military action as he opened the debate, Mr Cameron conceded that there could be no "100% certainty" about who committed the chemical weapons attack in Syria. The Prime Minister said there was not "one smoking piece of intelligence" but insisted he was convinced by the evidence that it was "beyond doubt" Bashar Assad's regime was responsible.
Meanwhile, senior Labour MP Jim Fitzpatrick resigned as a shadow minister after saying he would vote against Ed Miliband's policy on Syria. He quit after telling the Commons he was "opposed to military intervention in Syria, full stop".