Downing Street party: How Boris Johnson's future hangs in the balance

The prime minister will face Sir Keir Starmer again in the Commons at lunchtime.

Boris Johnson will know, though, that some of his fiercest critics will be sitting behind, and not in front of, him.

Half a dozen of his own MPs have publicly called for him to go, and some others are privately of the same view.

He will, of course, expect to hear cheers from supporters and from some other colleagues who want to rally round in the face of Labour attacks.

Perhaps more crucially, though, are a disparate group of MPs that you might call "considerers". They are weighing up whether to submit letters of no confidence, which would trigger a leadership contest.

They will be listening closely to his answers.

So what - for now - is holding them back?

Most Conservative MPs will say they will await the conclusion of the investigation into the gatherings in Downing Street by the senior civil servant Sue Gray before deciding how to proceed.

One of those who has already called for Mr Johnson's departure has urged his colleagues "to grow some". He said he had 1,500 hostile letters from constituents and "this can't go on".

But in truth some of those colleagues tell me they are merely awaiting the completion of "due process".

Virtually irrespective of what Sue Gray says, they will join the cacophonous chorus calling for him to go in due course.

One of them said he was simply waiting for other colleagues to "catch up" with the conclusion he had reached.

For others, though, it matters hugely what Ms Gray says.

Although she won't sit in judgement of the prime minister, and will simply set out the facts, it will be important to some leadership-sceptics which facts she sets out.

One of them told me that if she can substantiate Dominic Cummings's claim that the prime minister was told about the nature of the event on 20 May 2020 - that was a social not a work gathering - then more letters of no confidence were likely to go in as MPs would feel they had been misled by the prime minister.

This, of course, may not materialise - the prime minister himself denied again on Tuesday that he was warned the event could be against the rules.

Dominic Cummings has alleged in his blog that the prime minister had been told.

This, too, puts pressure on Sue Gray to investigate the claims, and avoid any impression of a whitewash.

That said, if Sue Gray merely uncovers evidence that the civil servant Martin Reynolds was warned about proceeding with the event, some MPs will be content to see a clear-out of No 10 officials, and their potential rebellion could fade.

One well-placed MP declared "she is not going to throw the prime minster under a bus" - and that a plan was in place for staffing changes at No 10.

And a former cabinet minister told me he expected "deputy heads to roll" in order to save the prime minister. He did not believe Mr Johnson could remain in No 10 until the next election, but that he could stave off an immediate threat.

For some Conservative MPs there are other considerations.

One MP told me he was on the verge of sending a no-confidence email, but key figures in his constituency association - the grassroots Tories who selected him and campaigned for him - wanted to support the prime minister - and, more widely, there is polling which suggests he has retained more support amongst activists than voters.

However, there is significant pressure from Conservative voters.

At least one constituency association has been checking negative letters and emails against the list of supporters that they would canvass at election time, and that has made grim reading.

Many MPs say the incoming criticism goes beyond "the usual suspects". And this in turn could yet tip the balance against the prime minister.

So far cabinet ministers are staying solid. But the thin blue line of junior ministers is under strain.

Junior minister Maria Caulfield posted on Facebook that she "fully appreciates the depth of anger on this issue" amongst her constituents and that "it is clear the spirit of the rules" was broken.

Science minister George Freeman made clear that he was angry about staff in No 10 holding parties - though he would not pass judgment at this stage on the prime minister.

And privately another junior minister has said that her postbag isn't just critical of the prime minister but increasingly hostile to her. She is not - even off the record - calling for the prime minister to go yet - but the prospect of "party-gate" sullying the reputations of those who had nothing to do with the gatherings has touched a raw nerve.

If the prime minister's future is in the balance, this may in due course put more weight on the "go" rather than "stay" scale.

It's been suggested that some sceptical MPs will rally round because they do not want to indulge a "Dominic Cummings coup" against an elected leader.

And that this could perhaps move the balance back in the PM's favour.

But that he is the source of some - and by no means all - of the allegations against the PM isn't weighing heavily on the minds of some of the considerers.

One of the 2019 intake was under no illusions about Mr Cummings' role, but nonetheless felt that his attacks would have misfired if the prime minister himself had not given him the ammunition.

A former minister told me: "Just because he is the source of a story is not a reason to ignore the story."

And there are concerns that Mr Cummings will come forward with more claims even after Sue Gray reports, and this will be corrosive if the prime minister stays in No 10.

So far it's been fellow Brexiteers - such as Andrew Bridgen and Tim Loughton - who have publicly urged the prime minister to go.

Worryingly for him, these also tend to be people sceptical of lockdown measures.

So one aspect of his fightback - the lifting of restrictions in England later this month - won't suddenly get them to reverse their view.

But what of those who were perhaps more sceptical of Boris Johnson all along?

The One Nation group of MPs met behind closed doors on Monday night.

They had gathered to hear from the DUP's Sir Jeffrey Donaldson on Northern Ireland - a party which has had its own leadership problems - and the question of Conservative leadership came up in the second half of the meeting.

MPs shared tales of dismal postbags and I'm told there was an "intense" discussion on what to do.

While broadly there was a feeling of "inevitability", there were different views on timing.

One MP told me he was against triggering a confidence vote now, because he feared "Boris Johnson could win" and "we would be stuck with him for the next two years".

But one of his colleagues favoured a swifter move because he felt that unless Mr Johnson won a confidence vote convincingly "he will be handed the pearl-handled revolver".

How the timing of a contest would play with the public is weighing on some people's minds - would it look less indulgent to move after potentially poor local elections in May rather than in the midst of a pandemic?

And would it be better if Mr Johnson took the hit for national insurance rises and increased energy costs in the spring rather than his successor?

"It's all about the pace at which this occurs" - and not whether it will occur - according to a former minister.

But some MPs are already mulling over what would come next.

One MP who has not been publicly critical of the prime minister is simply concerned that any replacement would not be as effective at appealing to first-time Conservative voters.

"Do we really want a dull manager as leader?" he mused.

And those who are likely to seek to replace him do not seem to be in an immediate rush.

Letters of no confidence are sent to Sir Graham Brady - the chairman of the backbench 1922 committee.

He is the very personification of discretion so estimates of how many letters he has received - 54 would lead to a confidence vote, which in turn would trigger a leadership contest if the prime minister loses that vote - are hugely speculative.

One former minister believes the sheer volume of public complaint makes it likely that more letters may have been, or will be, quietly submitted by his colleagues without much flourish or co-ordination.

And just 24 hours before PMQs a group of around 20 MPs elected at the 2019 election met behind closed doors at Westminster, where the mood was said to be hardening - with the possibility that more no-confidence letters are submitted after PMQs.

The Northern Research Group of MPs also met privately on Tuesday afternoon.

So it may take little more to tip the balance.

But, of course, letters can be withdrawn as well as submitted - so the prime minister's performance at PMQs, and then the tone of his response to Sue Gray, may yet tip the balance back in his favour.

And while Labour is publicly claiming it is in the "national interest" for Mr Johnson to go, privately shadow ministers believe it's in their party interest if he continues in office.

So if Sir Keir Starmer succeeds in further wounding the prime minister, it will be his own MPs who will have to decide whether it's better to then move in for the kill.