In the end the answer was absurd, as it was always going to be. Boris Johnson admitted in an explosive session of PMQs that he had attended the BYOB gathering on 20 May 2020 – but he thought it was a work event.

The PM knew he had to offer something. The pressure – from a furious public and his own truculent MPs – was just too great. So, with great solemnity, he conceded that he had been at the gathering in the Downing Street garden.

He said he took “full responsibility” and accepted that “things should have been done differently”. He offered a “heartfelt” apology.

But that was as much ground as the PM was going to give. He then reverted (repeatedly) to his first line of defence: that we should wait for the inquiry by Sue Gray – a senior civil servant who is looking into allegations of rule-breaking Christmas parties – to conclude.

What Johnson did not admit to was having broken the rules. “I believed implicitly that this was a work event,” he said. “Number 10 is a big department with the garden as an extension of the office.” His actions, he said, “fell, technically, within the guidance”.

Johnson’s finely calibrated and legalistic apology, as human rights barrister Adam Wagner points out, was to the millions of people who – wrongly, he implied – saw things differently. He was sorry they had got the wrong end of the stick.

Sir Keir Starmer came into his own in the Commons, posing all the right questions with precision and force while Johnson clung to his flimsy defence. MPs then tore into the PM with icy rage as he sat there, hangdog. The Teflon sheen was gone.

So what now?

By admitting he attended the party, the PM’s options for another Houdini-like escape have narrowed. The line that the party could be classified as a work meeting is unlikely to hold – the BYOB element is a smoking gun. Yet Johnson’s “I thought it was a work do” defence was calculated to leave him with a get-out if Gray concludes that the “event” was a party and, therefore, illegal. This could be his last resort. And ultimately, it comes down to how Gray defines a “work event” – a concept that doesn’t appear in any of the Covid rules.

As Sir David Normington, a Whitehall permanent secretary, notes, Gray is in a “very odd” position. “She will be very aware that she has the reputations and possibly the careers of senior civil servants and possibly of the prime minister in her hands. That is a very difficult position to be in, however fair and fearless and rigorous you are.”

A police investigation could delay the inquiry and offer Johnson a reprieve of several months. But if yesterday’s performance in the Commons is anything to go by, he is a diminished and desperate figure who increasingly looks on borrowed time.

Twitter: @mattiebrignal