Mandelson, previously ambassador to Washington, was not approved by the security service when it conducted a background check before his appointment, The Guardian reveals.
Despite that, he became ambassador and was fired last year when his close contacts with convicted sex offender Jeffrey Epstein were discovered.
Starmer claims he was unaware that Mandelson had failed the security clearance and has said in Parliament that everything went smoothly.
He should resign, says Conservative Party leader Kemi Badenoch, according to the BBC, and her call is supported by the Liberal Democrats, the Reform Party and the Greens.
