One of Labour’s most senior MPs last night demanded answers from the Foreign Secretary over the vetting process for Lord Mandelson’s appointment as ambassador to the US.

Emily Thornberry wrote to Yvette Cooper – who has been in the job for just a week – showing the strength of unhappiness about the affair even in the party’s own ranks.

As chairman of the Commons foreign affairs committee, Dame Emily said she ‘repeatedly’ asked to question Lord Mandelson before his departure for the US in February but was denied by the Foreign Office and Ms Cooper’s predecessor, David Lammy.

Dame Emily asked for clarification over what security concerns were raised in the vetting process and how the Foreign Office responded to those concerns.

She also asked whether the Foreign Office felt the concerns represented a ‘potential barrier’ to Lord Mandelson’s appointment and if any conditions were imposed on his remit.