The last time I visited the topic of a vote of no confidence in the Prime Minister was back on 19 January in the wake of the first shockwaves from allegations around parties held in Downing St during lockdowns.

At the time, the Pork Pie Putsch players (so-called because one of the ring leaders was the MP for Melton Mowbray), were counselled by older hands to await the outcome of the Sue Gray Report, which had been commissioned by the PM to get to the bottom of what went on behind the famous black door.

Well, now we know and, despite an initial narrative that Boris Johnson had ‘gotten away with it’, it appears that the report lit a slow fuse which has reignited the fire of public anger over the Jubilee weekend.

The trickle of letters to Sir Graham Brady, Chair of the 1922 Committee which oversees internal Parliamentary party business, has swelled in recent days and Sir Graham announced this morning that the number had ‘exceeded’ the 54 letters, or 15% of the Parliamentary Party, needed to trigger a vote of no confidence in Johnson.

So, what does this mean for the PM? A secret ballot will take place in the House of Commons this evening between 6.00pm and 8.00pm and the results will be known from 8.45pm. To win, the PM needs to secure a simple majority from MPs eligible to vote – and his team is letting it be known that if he wins by a single vote he will be staying on. The 22 Committee rules say that the contest cannot be re-run over the next 12 months. This would mean the PM would be safe from further challenges and will most likely lead the Conservatives into the next General Election in 2024.

The most likely outcome is that Johnson will secure the majority he needs, not least because the ‘payroll vote’ of 172 MPs (almost half the party) are in ministerial positions or Government roles and are bound by collective responsibility to vote in favour of the Government (although it will be a secret ballot). At the time of writing – mid-afternoon – 90 MPs had already publicly pledged their support.

However, size matters and all eyes will be on the strength of Johnson’s majority. Both Margaret Thatcher and Theresa May won confidence votes but didn’t survive their term of office. John Major won his confidence vote and, although he didn’t resign, was annihilated in the subsequent general election in 1997. Analysts predict that if Johnson fails to win with a solid two-thirds majority, he may still be in real trouble. What is key for the PM will be whether he can keep the number of rebels in double figures.

The timing of the vote coming in advance of the 23 June by-elections in Wakefield and Tiverton and Honiton also presents several challenges. Team Johnson was keen for the no-confidence vote to take place before the by-elections because defeat, which looks likely in these constituencies, would blow a hole in the argument that Johnson remains a vote winner.

However, the difficulty for MPs who support him is that they could find themselves stuck with a leader who has shown to be vulnerable in the red wall and amongst traditional Tories in the south and who has failed to draw a line under Partygate just as the Privileges Committee launches its inquiry into whether he knowingly misled Parliament. It may lead some to think if not now, when?