UK Prime Minister Keir Starmer is facing increasing calls to resign following his appointment of Peter Mandelson as ambassador to the U.S., despite Mandelson failing security vetting. This decision has drawn scrutiny after it was revealed that Starmer was not informed of the vetting failure until recently. Opposition leaders, including Ed Davey of the Liberal Democrats, are accusing Starmer of misleading parliament by suggesting Mandelson had passed the vetting process. Starmer is expected to clarify his knowledge of the situation in a parliamentary statement. The controversy raises questions about Starmer's leadership, especially as he has the lowest approval ratings of any UK Prime Minister. If evidence emerges that Starmer or his advisers were aware of Mandelson's vetting failure, it could lead to calls for his resignation, similar to past political scandals that resulted in resignations for misleading parliament. The Labour Party's rules make it challenging to remove a sitting Prime Minister, but a leadership challenge could be initiated if enough support is gathered for a replacement.