The heavy mahogany doors of Marlborough House remain shut, but the noise from inside is deafening. Commonwealth heads of government are in a closed session that will define the bloc's future. And it is not going well for the British side.
African and Caribbean leaders are demanding a formal apology for the transatlantic slave trade. Not just words. Reparations. A full-throated acknowledgement of historic wrongs. The UK delegation, led by the Foreign Secretary, is pushing back. Hard.
Sources inside the room describe a tense atmosphere. The Caribbean Caucus is united. African nations are backing them. They say Britain cannot host the summit and then dodge the central question of its imperial legacy.
Why now? The timing is no accident. The summit was meant to be a celebration of 'family ties'. A chance for the new government to reset relations after the Brexit years. Instead, it has become a reckoning.
No 10 is nervous. They know the polls at home show little appetite for massive reparations. But they also know a walkout by dozens of nations would be a disaster. A humiliation on the world stage.
The game is on. The UK is offering a 'meaningful dialogue'. A working group. A promise to 'listen'. But the Caribbean leaders are not buying it. They want an apology. Now. Not a committee.
One senior diplomat told me: "We have been listening for 400 years. It is time for action."
The Commonwealth Secretary-General is trying to broker a compromise. A statement that acknowledges the 'pain of history' without using the A-word. But the hardliners are not backing down.
What happens next? The session was supposed to end at 6pm. It is now 8pm. Tea and biscuits have been brought in. The real negotiating is happening in the corridors.
I am hearing that a small group of leaders, including the Prime Minister of the Bahamas and the President of Ghana, have been shuttling between the UK and Caribbean delegations. Trying to find a fudge. A way to save face.
But the gap is vast. The Caribbean nations want a specific apology, a commitment to reparative justice, and a timeline. The UK is offering a review of educational materials and a 'shared history' project.
One UK official described the Caribbean demands as 'unacceptable'. Another said the UK would 'not be lectured' on its record.
Yet the arithmetic is brutal. A majority of Commonwealth members are small states with colonial histories. They have the numbers. And they have the moral high ground, at least in the court of global opinion.
The UK's best hope is that a few moderate voices will peel off. But so far, solidarity is holding.
If the talks collapse, expect a joint statement from the African and Caribbean caucus. A statement that will dominate headlines for days. And a British government left explaining why it could not say 'sorry'.
The irony is not lost on anyone in this building. The Commonwealth is a voluntary association. It survives on goodwill. And goodwill is exactly what is being tested tonight.
For now, the doors remain closed. The negotiations continue. I will update as soon as I have a result.
This is Eleanor Rigby, Political Bureau Chief, at Marlborough House.