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Sometimes politics is subtle.

And then there are days like today.

Sitting in the Press Gallery in the House of Commons, watching Prime Minister's Questions, you could feel and hear Boris Johnson's authority draining away.

I was watching the faces and demeanour of Conservative MPs during the session.

Tribalism is measurable in decibels in Westminster, but today the Conservative benches sat in near silence.

The noise came from the opposition benches, in what was Boris Johnson's toughest PMQs since the general election. And it was still only lunchtime.

By early afternoon, the letters of resignation and no confidence were tumbling in, and even Mr Johnson's most loyal supporters privately - and often bluntly and colourfully - acknowledged the game was up, it was over.

Is it curtains? I asked a cabinet minister.

"Yes I fear so. It's hours and days," came the reply.

Then, one after another, cabinet ministers texted me saying they were heading to see the prime minister this evening to tell him explicitly he had to go.

Westminster is a postcode defined by power. And tonight it is shifting.

The end of Boris Johnson's premiership appears imminent.