Why has Sue Gray's report not yet appeared?

The expectation across Whitehall is that Sue Gray's long-anticipated report would be passed to Downing Street on Wednesday.

So why hasn't it appeared, and what is it likely to find when it does?

The announcement of the Met Police investigation has inserted a spanner in the works - especially as all contentious evidence had been given to the police by the cabinet office last week.

And there had been liaison throughout the internal inquiry.

It's understood the Met Police Commissioner Cressida Dick examined the evidence at the weekend.

While the Met has not objected to the report being published - since any action they take could be limited to issuing fixed penalty notices, and not lead to a criminal trial - Downing Street has said reassurances were now being sought that none of the content would cut across police work.

I am told by Whitehall sources that Sue Gray is determined that the report she submits to Downing Street will be in a form that can allow for it to be published by No 10 in full.

What she wants to avoid are last minute redactions, or a report that focuses on the least egregious events, while leaving everything that's contentious to the police.

Sue Gray has built a formidable reputation within the civil service and will not want undermined by allegations of a whitewash, or a less than thorough job.

So it is perfectly possible that minor redrafting or reworking could be undertaken which achieves the twin aims of ensuring the police would have no last minute objections, while not skirting round some of the findings about more controversial gatherings in government buildings.

According to sources close to the Gray team, there has not been any political interference to adjust the content, so that is not the cause of delay.

The prime minister will certainly see the full report before it is published but will not have a veto over its content.

And there has been no attempt by civil service trade unions to delay the report or ask for retractions.

Only very senior civil servants will be named and those who will be named have already been informed.

But it's not impossible that more junior staff individually may have raised concerns that while they are not going to be identified, the information in the report might make them nonetheless too easy to identify.

Insiders also maintain that while the whole process has been a difficult one for Ms Gray, she would push back against any informal attempts by other senior civil servants to alter the tone of the report, or to dilute the findings.

According to those who know Sue Gray's track record - as the former director general in charge of propriety and ethics in Whitehall - she holds her fellow civil servants to very high standards.

From those close to the process, it would be difficult for her to conclude that Downing Street is being well run.

While she cannot recommend any action be taken against politicians, she can be very critical about the civil service leadership in Downing Street, and of the culture in the building.

She is unlikely to mince words, and her report could subsequently trigger disciplinary action against some staff.

It will almost certainly make difficult reading for senior people - including some who have so far not been mentioned prominently in public.

Of course, there is little doubt that supporters of Boris Johnson would find it convenient to blame the advisers and indeed the longer any delay, some of those supporters may go on the front foot and call for reforms in the operation.

As for the prime minister, the terms of reference which Sue Gray was given may work to his advantage.

As head of propriety and ethics, when she dealt with complaints against ministers she would form a judgment on whether the minister or the complainant was the more reliable witness.

But as she has been restricted now to establishing the facts, she will reach no such judgment.

So, for example, if the former aide Dominic Cummings told her that Boris Johnson was warned that the 20 May 2020 gathering was a rule-breaking social event and the prime minister maintains he was not, she will not form a judgment on who is most likely to be telling the truth.

Of course, if she were to have written evidence in the form of a Whatsapp or email message read by the prime minister, simply setting out the facts could be damaging for Boris Johnson.

But those close to the 'save Boris' operation aren't expecting a 'smoking gun' - instead, they say, it's more likely there will be 'contestable' claims.

While the prime minister hasn't seen the report, he will have a sense of where it is going through the interviews he and staff have given to the inquiry.

Those who expect very firm conclusions - especially where politicians are concerned -are likely to be disappointed.

And those internal and external opponents of Boris Johnson slavering at the prospect of Whatsapp messages and potentially embarrassing photographs from inside Downing Street being made public are likely to remain unsated.

Sue Gray won't be publishing all the evidence she has received, or handed to the police.

But those close to the process say it could be that some of the evidence could be incorporated verbatim into the report where it fits the narrative and perhaps provides a salient example of the prevailing culture.

Ultimately any decision on law-breaking is for the police, not for Sue Gray -so it is perfectly possible that hers will not be the final word in this political drama.