LORD Peter Mandelson’s appointment as ambassador to the US was “high risk, high reward,” a Cabinet minister said today.

The Scotland Secretary Douglas Alexander told broadcasters that Lord Mandelson’s appointment was a “judgement” that an “unconventional presidential administration” required an “unconventional ambassador.”

Sir Keir would also not have appointed the so-called “prince of darkness” if he had known the extent of his relationship with the paedophile Jeffrey Epstein, the minister told BBC Breakfast.

Lord Mandelson was sacked as Britain’s ambassador to the US on Thursday after emails emerged in which the peer offered support to Epstein even as he faced jail for sex offences.

Sir Keir, who had said he had “full confidence” in Lord Mandelson before the emails were published, now faces questions over what he knew and when about the ex-ambassador’s ties to the notorious sex trafficker.

Speaking to BBC Breakfast on this morning, Mr Alexander said that “nothing justifies” Lord Mandelson’s appointment “in light of what has now emerged.”

Questions about what the Prime Minister knew of Lord Mandelson’s connection to Epstein come after allies of the peer told The Times that he had admitted in his vetting interview that he continued his relationship with Epstein for many years.

Labour backbenchers have expressed anger at Sir Keir’s handling of the row over Lord Mandelson, with almost 50 reported to have contacted chief whip Jonathan Reynolds calling for the peer to be sacked. Left MP Apsana Begum pointed out that she has still not had the Labour whip restored for having rebelled against the two-child benefit cap, when Lord Mandelson retains it despite having ties to a sex offender.