The UK Government is preparing to publish a second and substantial tranche of documents relating to the appointment of Labour peer Lord Peter Mandelson as US ambassador, following a parliamentary demand under a humble address procedure.

The release is expected to be one of the largest ever made to the House of Commons, containing hundreds of files, including messages between Lord Mandelson and government ministers as well as senior advisers such as Morgan McSweeney, the Prime Minister’s former chief of staff.The publication follows ongoing controversy over Mandelson’s appointment and concerns about the vetting process.

MPs had previously voted to compel the Government to release a wide range of materials after questions were raised about security assessments and transparency.

A first set of documents released earlier in the year revealed that Sir Keir Starmer had been warned of reputational risks linked to Mandelson’s association with Jeffrey Epstein.

Reports also indicate that Mandelson failed security vetting due to concerns over his links to figures in China, Russia and Israel, although his appointment was ultimately approved after Foreign Office intervention.Senior official Sir Olly Robbins reportedly overruled security advice and was later removed from his position.

Some materials, including the UK Security Vetting summary, are expected to be withheld at the request of the Metropolitan Police to avoid interfering with an ongoing investigation into alleged misconduct in public office.

The Government insists it is fully complying with parliamentary demands and emphasises transparency, while opposition figures have accused ministers of excessive redaction and potential concealment.

The unfolding release is expected to intensify political scrutiny of both the vetting process and the handling of sensitive governmental appointments.