Ministers braced as Mandelson document release will expose government working

The appointment of Peter Mandelson as the UK's ambassador to Washington is like a bad news boomerang for the government.

Over and over again the prime minister's most consequential judgement in office circles back into ministers' airspace – and today it will do with gusto.

It will be "another one of those weeks" one senior figure said, wearily.

The scale of what is to come will be quite something: the largest government publication ever put before the Commons, and therefore us, other than the Chilcot Inquiry into the Iraq War – and that was a 12 volume, 2.6 million word affair.

Printed and bound, the publication will be made up of three volumes. More than 160 of the pages are made up of Lord Mandelson's text messages and WhatsApps.

The bundle will include a substantial explanation from the government of how much effort it took for officials to collate all the information parliament required the government to release, describing it as thousands of hours of work from officials.

In big picture terms, the documents will offer a fascinating internal insight, at scale, into how government works: the private interactions, information flows and disagreements.

Those with an understanding of how the embassy in Washington works describe it as being almost like a government department itself in size – and, crucially, with connections to so many aspects of the Whitehall government machine, given the importance of the UK-US relationship. This is particularly true on military and intelligence matters, much of which will likely be redacted from this document drop on national security grounds.

Folk in government are braced for the inevitable awkwardness of exchanges that they had assumed at the time would be forever private being catapulted into the light of day. Many of the messages are expected to reflect the reality that, at the time, Lord Mandelson was seen as one of the most influential powerbrokers in the Labour Party.

"Excruciating", "sycophantic" and "cringeworthy" are the words being used to describe some of them. Let's see.

The very human tone and tenor of messages – particularly on WhatsApp where text is so often a substitute for in the moment verbal conversation – is likely to stand out.

We understand a good number of cabinet ministers were lavish in their praise of Lord Mandelson immediately after he was sacked. Could there be some comparing and contrasting to be done between those messages, if they are in this tranche, and what any of those ministers have said publicly about him since? And what about criticisms made of the prime minister?

We are not expecting to see the vetting file compiled prior to the decision to send Lord Mandelson to Washington.

The Guardian has reported that the former ambassador's associations with senior figures in China, Russia and Israel were among the concerns raised by the vetting agency.

In April, Sir Olly Robbins, the former head of the Foreign Office, told MPs that he granted Lord Mandelson security clearance to take up his post as ambassador having put "mitigations" in place.

But it is not expected that today's documents will include any record of any measures taken to mitigate any security concerns.

The BBC understands Lord Mandelson doesn't believe there were any security concerns, wasn't asked to do anything to deal with any and there has been a muddling of key details relating to these claims.

There were separate concerns raised in the process about potential commercial conflicts of interest arising because of clients of Lord Mandelson's now defunct consulting firm Global Counsel.

These were dealt with by the deputy head of mission in the embassy overseeing any dealings with these companies, we are told.

By the end of today, from Downing Street's perspective, could they be through the worst of all this?

Yes, probably – but it won't be entirely over.

The police investigation into Lord Mandelson continues. He has repeatedly let it be known that he believes he has not acted criminally, did not act for personal gain and is cooperating with the police.