The ex-civil servant behind the decision to approve Peter Mandelson’s appointment as British Ambassador to Washington says he felt political pressure to rush through the appointment despite security concerns.

Olly Robbins, former head of the Foreign Office, said those concerns did not relate to Mr. Mandelson’s relationship with the sex offender Jeffrey Epstein. He declined to say when questioned by lawmakers what led the government’s vetting agency to flag Mr. Mandelson as a potential security risk.

Mr. Robbins said the vetting agency considered Mr. Mandelson a “borderline case” and was “leaning toward recommending against” giving him security clearance.