Keir Starmer activated submarine mode today as he braces for a huge dump of ‘cringe’ messages between ministers, aides and Peter Mandelson.

No10 has confirmed the PM will not be speaking publicly as the Government finally publishes a thousand pages of incendiary material covering the New Labour architect’s disastrous appointment.

Instead his right-hand man Darren Jones is due to face MPs’ questions about the revelations this afternoon. 

The messages are believed to include indiscreet criticism of the embattled PM by his own senior team. Sir Keir himself is thought to have sent Mandelson a gushing text saying he would be ‘brilliant’ as US ambassador.

However, some key papers are still being withheld at the request of Scotland Yard, which is investigating the former peer over alleged misconduct in public office.

The Government was forced to agree to the disclosure by a massive Labour revolt in February – which almost saw Sir Keir kicked out of No10.

Some insiders fear that Andy Burnham could seek to exploit the premier’s woes for his leadership bid by claiming the Mandelson documents show Westminster is ‘broken’. 

Touring broadcast studios this morning, Health Secretary James Murray said: ‘I think the level of transparency is going to be unprecedented.

‘The volume of information that’s going to be put out is unprecedented.

‘It’s right we do that. We have been very clear that the appointment of Mandelson was wrong.

‘Parliament then decided that this information will be made public. The Government is fully complying with that, and it’s important that we honour that commitment to transparency.’

In February MPs backed an archaic parliamentary procedure known as a ‘humble address’ to demand the papers.

The vote followed the launch of a police probe into Mandelson, and the revelation that Sir Keir went ahead with the appointment despite vetting officials raising concerns.

A first tranche of documents, released in March, showed Sir Keir was warned in a due diligence document about Mandelson’s long-standing links to paedophile financier Jeffrey Epstein.

The second set of documents is expected to include messages between Mandelson and ministers and government advisers, including Sir Keir’s former chief of staff Morgan McSweeney.

It is not clear how many of Mr McSweeney’s exchanges with Mandelson have survived, after his mobile phone was stolen last year.  

Former health secretary Wes Streeting has already published his own messages with Mandelson, including discussion of Gaza and criticisms of Sir Keir’s leadership.

But the batch is not set to include Mandelson’s UK Security Vetting (UKSV) summary after the Metropolitan Police asked the Government to withhold the document.

UKSV recommended against granting Mandelson security clearance, but top Foreign Office official Sir Olly Robbins overruled that advice.

Sir Olly was effectively sacked in April after it emerged he had overruled the advice, with the Prime Minister insisting he was unaware of UKSV’s recommendation.

The Government has said it has only withheld documents where requested by the police, and all other documents will be published.

Several documents have also been redacted, either to remove personal details such as the names of junior officials, or on national security grounds.

Redactions on national security grounds were agreed with Parliament’s Intelligence and Security Committee following a compromise with MPs to head off a backbench rebellion on the demand for documents.

Last month, the committee raised concerns that the Government was applying redactions ‘too broadly’, and cited the UKSV vetting file as an example of documents being withheld without Parliament granting ministers the ‘authority’ to do so.

Lord Beamish, the committee’s Labour chair, insisted this morning that nothing had been struck out to avoid ’embarrassment’. 

‘The committee has taken a very robust line in terms of getting the balance right between protecting national security, but also ensuring that Parliament and the public see as much as possible,’ the former minister said.

The Conservatives have accused the Government of seeking to perpetrate a ‘cover-up’, with shadow minister Alex Burghart writing to the chief secretary to the Prime Minister to demand ‘full transparency’.

Mr Burghart said: ‘I will remind you that the House did not provide the Government with the choice about redactions beyond the narrow scope in the humble address.

‘This behaviour will be viewed by the House as a contempt of Parliament, and as a cover-up by the British public.

‘Even if the Prime Minister leaves office in a few weeks, I can assure you we will hold you to account for this.’

The Prime Minister’s official spokesman said: ‘This is a task that has involved every Government department.

‘The result is the largest-ever Government response to a humble address. It represents thousands of hours of work from officials across the Government to deliver an unprecedented piece of Government transparency.

‘Our guiding principle throughout has been to comply as transparently and as swiftly as possible.’

He added: ‘For example, material of a party political nature will be included in the publication, which is contrary to usual practice and precedent, in order to demonstrate the maximum possible transparency.

‘A number of documents have also been declassified to enable publication.

‘In order to provide a transparency to Parliament and the public alongside the documents, we will also provide a clear explanation of the steps taken in this official-led process to gather the documents and the approach to any redactions.’

A Metropolitan police spokesman said: ‘An investigation into alleged misconduct in public office is underway and it is vital due process is followed so that our criminal investigation and any potential prosecution is not compromised.

‘We are working with the Cabinet Office to review relevant documents provided to us by them.

‘While complying with the Humble Address is a matter for government and parliament, we have asked the Cabinet Office not to make some documents public as they would have a detrimental impact on our investigation or any subsequent prosecution.’

Mandelson has denied any wrongdoing.