A senior civil servant who has worked with Cabinet Secretary Simon Case called a radio phone-in to warn of No10's "dangerous lies".
The public servant, who called James O'Brien's LBC show under the name 'Stewart,' urged Downing Street press officers to "get up and walk away from your desks" because "democracy is in danger".
Boris Johnson is facing accusations of a “cover up” over his appointment of Chris Pincher after a former senior official said the Prime Minister was briefed “in person” about an investigation into his conduct.
Sir Simon McDonald, known for having the "utmost integrity", sources told the Mirror, suggested No10 is lying about what happened - and what the Prime Minister knew.
"No10 keep changing their story and are still not telling the truth", he blasted in his letter, adding, "They need to come clean. The language is ambiguous,
"It's sort of telling the truth and crossing your fingers at the same time."
Since Mr Pincher's resignation last week, No10 has been accused of shifting its account of what Mr Johnson knew of his past conduct when he made him deputy chief whip in February.
He dramatically quit over allegations he drunkenly groped two men at the Carlton club - often described as the heart and soul of the Conservative party.
Reacting to Sir Simon's letter, Deputy Prime Minister Dominic Raab told the BBC he did not know if the Prime Minister had been breifed about the inquiry.
“That’s news to me, I wasn’t aware of that. It’s not clear to me that that is factually accurate.”
He added: “I have discussed this with the prime minister over the last 24 hours. It is not my understanding that he was directly briefed.”
The anonymous senior civil servant told LBC: "I felt compelled to ring in today because Raab's comments were 'absolutely extraordinary'.
"A serving cabinet minister, trying to undermine the integrity of a longstanding senior civil servant who has come out and put his head above the parapet... that is a big deal.
"What can motivate cabinet members to trot out lies? I believe the know in their heart of hearts their careers are over."
The civil servant also urged Simon Case to step down and resign.
"After seeing Simon's letter, how can you possibly continue your role as Cabinet Secretary and head of the civil service? Knowing the guy you are working for is overseeing one of the most corrupt Government's in modern history?"
