The most senior official at the Foreign Office is leaving his post, after the decision to fail Lord Mandelson’s security vetting for the role of US ambassador was overruled by his department.
The BBC understands the prime minister and the foreign secretary have lost confidence in Sir Olly Robbins.
A spokesperson said neither Sir Keir Starmer nor any minister were aware Lord Mandelson had failed the vetting process until earlier this week.
Lord Mandelson was sacked as ambassador last year over his ties to the late convicted sex offender Jeffrey Epstein.
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