The Foreign Affairs Committee has concluded that Downing Street 'forced' the appointment on the Foreign Office and that the process was 'being made up as it went along'.
It has emerged that seven people, plus an unknown number of security officials, found out before the Prime Minister that red flags had been raised during background checks on the US Ambassador.
King Charles and Queen Camilla rounded off a packed and diplomatically crucial first day in the United States by hosting a lavish garden party for 650 guests.
The Prime Minister had already been accused of wrongly assuring MPs that full due process had been followed in the appointment of the Labour grandee as US Ambassador.
Keir Starmer admitted he was told on Tuesday evening that Peter Mandelson had been appointed US Ambassador despite failing his security vetting - but kept quiet for a day-and-a-half.
As many as 100,000 records and messages are believed to have been gathered about the Labour grandee's appointment as US Ambassador and his sacking over his friendship with Jeffrey Epstein.