Exclusive: Officials have spent weeks debating whether or not to release highly sensitive information about the affair

Keir Starmer was left in the dark about sensitive information relating to Peter Mandelson’s security vetting by two other top civil servants, including the head of the civil service, the Guardian can reveal.

The prime minister said on Friday that it was “unforgivable” and “staggering” that senior officials did not tell him that Mandelson failed a security vetting process weeks before he took up his role as ambassador to Washington.

Exclusive: Adrian Fulford, who chaired the recent Southport inquiry, will also examine the wider national security vetting system

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A retired high court judge is expected to review Peter Mandelson’s vetting process and the wider national security vetting system.

The review, commissioned by Downing Street, comes after a Guardian investigation revealing that security officials decided Mandelson should not receive developed vetting clearance, but were overruled by the Foreign Office to allow him to become US ambassador.