Incensed by what Ed Miliband had just told David Cameron about Labour's revised stance on Syria, a disbelieving Whitehall source blasted: "No 10 and the Foreign Office think Miliband is a f****** c*** and a copper-bottomed s***."
The cause of this explosive comment was a phone call at 5:15 precisely on Wednesday evening when, after a number of face-to-face meetings with the Prime Minister, Miliband told Cameron of Labour's new conditions for its support. The crimson tide rose up.
Just 24 hours earlier, the Leader of the Opposition had held talks with the Prime Minister and his deputy, telling them bluntly: "You have to realise that after Iraq nobody trusts any of us."
Miliband later emerged from Downing Street to announce that Labour would consider backing military action so long as it was "legal, specifically limited to deterring the future use of chemical weapons and any action contemplated had clear and achievable military goals". The letters "U" and "N" were absent.
After chairing a meeting of military chiefs and senior ministers to map out Britain's contribution to the allied strike, Cameron began drawing up a Commons motion he believed would gain cross-party support. He thought he had Labour's backing in the bag and could tell Barack Obama how Britain would, as usual, stand shoulder to shoulder with America. The timetable of a weekend strike on Syria was in place. The last piece to fall in place was the Commons vote.
Nick Clegg stressed how there had to be a "UN moment". So Britain sought to get a fresh mandate, authorising "all necessary measures" under the United Nations Charter, knowing full well Russia would veto it.
But between Tuesday and Wednesday, some of Miliband's colleagues expressed deep unhappiness with his benign acceptance of the PM's line on Syria; his support had been won "too cheaply".
Senior colleagues questioned the headlong rush to action, pointing out that the United Nations inspectors were still doing their work and should be allowed the time to complete it. Diane Abbott threatened to resign from the frontbench; others could well have followed.
So in that "bad-tempered" phonecall on Wednesday, Miliband set out Labour's new conditions: the UN inspectors had to report back; there had to be compelling evidence the Assad regime was responsible for last week's chemical attack in the Damascus suburbs - and there had to be a second vote to sanction any British military action.
The PM hit the roof and supposedly accused Miliband of "letting down America" and "siding with Lavrov" - the foreign minister of Russia, the Assad regime's loyal ally.
Within an hour Labour was briefing the press about its amendment; an hour after that Number 10 put out the Coalition motion, including the Opposition's demands on the UN and, crucially, the second vote.
Labour HQ insisted it was doing the right thing. Tory HQ was adamant Cameron was bending over backwards to present a united front.
But any sense of cross-party consensus evaporated as the Labour leader was accused of "flip-flopping" and playing politics.
As the crunch vote neared, all the party leaders had private meetings with their troops. Conservative leadership sources seemed buoyed that their leader had done enough to placate unease on the backbenches as the lessons of Iraq were raised time and time again.
Ahead of the eight-hour debate, Whitehall took the unusual step of publishing its legal advice - Britain could bypass the UN and intervene in Syria on humanitarian grounds - and a summary of what the Joint Intelligence Committee had discovered: that it was "highly likely" the Assad regime was responsible for the Damascus chemical attack - but it wasn't "compelling evidence".
The Commons debate was impassioned. Cameron gave an at times emotional speech, referring to the "horrific" videos in circulation, showing rows of dead children packed in ice.
Crucially, perhaps, he admitted there would never be "100% certainty" about culpability for the chemical attack and that, in the end, it was a judgment call.
The mood in the chamber was one of deep scepticism as MP after MP made their contributions, limited to five minutes.
Miliband, speaking in a low, priestly voice, insisted that while he was not opposed to military action, the PM had simply not made a good enough case. Tory MPs shuffled uncomfortably in their seats. One, Nadhim Zawahi, openly accused the Labour leader of seeking to divide the House for party advantage. "Where," he demanded, "is the national interest?"
Outside the chamber, Number 10's fury with the Labour leadership spilled over, with one of Cameron's aides even suggesting the Opposition was "giving succour" to the Assad regime. After the lengthy debate, the atmosphere grew tense as the voting began. After Labour's amendment was easily defeated, by 112 votes, Coalition shoulders eased as expectations rose that Cameron had won the day.
But the lines going into the No lobby for the vote on the government motion suddenly raised alarm among the Coalition whips.
When the vote was read out, confirming the Coalition had lost by 13 votes, the Labour cheer was mirrored by a sense of angry disbelief on the frontbench.
Justine Greening, the International Development Secretary, looked red-faced as she anxiously spoke to the Speaker. It later transpired she had missed the crunch vote, together with another minister, Mark Simmonds, because they had been in a side room discussing, apparently, Rwanda and had not heard the normally blaring division bell. John Bercow shrugged his shoulders.
The PM stood up to accept the will of Parliament, declaring: "We get it." A withering stare across the despatch box lingered.
The next day, it transpired that 10 ministers had missed the vote, as had a number of Coalition MPs - some were still on holiday.
If they had turned up, the Coalition would have won its vote. Sir George Young, the Chief Whip, was fingered by his colleagues, who insisted he had to "carry the can" for the Commons "shambles". His name is expected to feature high in those for the chop in a forthcoming Cabinet reshuffle expected soon.
The day after the night before, Cameron tried his best to appear collected, admitting: "Politics is difficult."
But when asked if he felt betrayed by Miliband's decision to withdraw his support, the PM would only say it was a matter for the Labour leader to "defend the way he behaved and his conduct". Translated: "Damn right".
As the fall-out to the Government's historic defeat was analysed about Britain's place in the world and the Conservative leader's place in his party, the PM had to make that phone call to Washington.
While there was, apparently, no formal apology, Cameron sought to make the best of a bad job by saying Britain remained committed to pushing for a robust response on Syria. Obama, ever polite, ended the conversation by saying: "OK, brother".
Over in Washington DC, John Kerry, the US Secretary of State, in making clear America was preparing for a surgical strike on Syria, gave a roll-call of America's friends, topped by the French "our oldest ally". Britain was not mentioned.
Later this week, world leaders will gather in Russia for the G20 meeting. When Cameron has to shake the hands of Obama and Vladimir Putin, it will be clear who will have the biggest smile.
But this weekend, as the UN weapons inspectors hand over their report, the world holds its breath.