In the letter, sent on November 29 and seen by The Sunday Telegraph, Matthew Jury, Mr Murray’s solicitor, said: “The original decision by Her Majesty’s Government to withhold key materials was arguably a manifest and improper interference with the due course of justice.

“In any event, it is reasonable to say that this decision must now be revisited; if only because any prior interference is harder to justify – if it could ever be justified at all – in light of the High Court’s ruling.”

Mr Murray reminded Mr Johnson in the letter that he had described WPC Fletcher’s killing as “cowardly” and had told MPs in the House of Commons last year that this was “a very important subject”.

Prime Minister Johnson had also committed to “see what we can do to take the matter forward” in the Commons, and then agreed in a private meeting with SNP MP Allan Dorans to look again at the Crown Prosecution Service’s original decision.

Mr Murray also asked in the letter to confirm whether Mabrouk had been handed “a ‘comfort letter’ equivalent to those given by the Blair government to IRA ‘On-TheRuns’ following a secret agreement with the IRA/Sinn Fein in 2000”.

The letter added: “Mr Murray respectfully asks if Her Majesty’s Government would now confirm whether the Blair Government did indeed issue Mabrouk with any form of comfort letter and, if so and as with the IRA comfort letters, has this now been rescinded?”

It concluded: “It was recently reported that the US and Libya were working together to extradite a suspect in the 1988 Lockerbie bombing. It is not too late to do the same for the suspects in Yvonne Fletcher’s murder.” Mabrouk is understood to be currently living in Libya, the letter added.

Senior counter terrorism detectives visited Libya to investigate the murder of WPC Fletcher at the end of last month’s trial “to discuss how to proceed with the investigation”, Libyan television said.