Mandelson was announced as the government’s choice to be US ambassador in December 2024, with developed vetting beginning the same day.
He formally took up the post in February 2025, before being removed last September when further details about his past ties with convicted sex offender Jeffrey Epstein came to light.
Sir Olly had been the most senior civil servant at the Foreign Office for two weeks when it was announced Mandelson had been picked for the Washington role.
Dame Emily Thornberry wrote to Sir Olly on Friday requesting that he appears before the Foreign Affairs Committee she chairs to answers questions.
BBC News understands he has not formally accepted the committee’s invitation, but friends of his said he was preparing to appear on Tuesday.
They also cast doubt on whether Sir Olly ever actually saw the recommendation to the Foreign Office not to approve Lord Mandelson’s vetting.
Allies said it was unlikely he was given the full report and may only have seen more specific risks thrown up during the process.
BBC News understands the vetting report returned a “no” verdict on whether a security clearance granting access to sensitive government material should be given to Mandelson.
Vetting of government employees and appointees is carried out by a government agency overseen by the Cabinet Office, which reports back to hiring departments with recommendations.