Officials at No 10 and the Foreign Office were aware of supportive emails between Lord Mandelson and paedophile Jeffrey Epstein when the prime minister initially defended the former ambassador on Wednesday, the BBC understands.
Sources stressed Sir Keir was not aware of the contents of the emails when he stood by Lord Mandelson at Prime Minister's Questions.
The BBC understands that a media enquiry outlining details of the messages between the pair was sent to the Foreign Office on Tuesday, and passed on to No 10.
Sir Oliver Robbins, the permanent under-secretary at the Foreign Office, asked Lord Mandelson about the emails on Tuesday, but did not receive a response until the following day.
Prime Minister's Questions takes place every Wednesday at midday while the House of Commons is sitting, with the prime minister facing questions from the leader of the opposition and other members of the House.
