The Bishop of Buckingham has told Boris Johnson to resign, saying he "obviously" lied about Partygate.
Church of England Bishop the Rt Rev Dr Alan Wilson said the Prime Minister's claims he did not realise what was going on were "nonsense" and that the country needed a leader it could trust.
His intervention came on a day where Mr Johnson was booed by some in the crowd as he arrived with his wife, Carrie, to attend the national service of thanksgiving for the Queen at St Paul's Cathedral.
Tory MPs who have been pressing for the Prime Minister to stand down following the final report by Sue Gray into lockdown violations in Whitehall appear to have decided to stay quiet during the Platinum Jubilee celebrations.
But asked on Times Radio if Mr Johnson should resign, Dr Wilson said: "The only answer is yes. I'm an Army baby and what they used to say in the Army was you can trust anybody, but you can't trust a liar.
"In all sorts of contexts, you have to be able to trust the people who lead you, who represent you."
He said the Prime Minister's excuses - "oh I didn't realise, I was ambushed by a cake" - were clearly nonsense and that he was "obviously" an out and out liar.
"Actually, you can see it from a mile off, but it's all nonsense. And most ordinary people realise it's all nonsense. It's not the parties actually. It's the lying. I think that's the problem," Dr Wilson said.
"I mean, everybody makes mistakes. And I think people are very tolerant about that. But I think it's very difficult to trust a liar."
A steady stream of Conservative MPs has being coming forward to call on Mr Johnson to quit since Ms Gray's report at the end of last month exposed the disregard for the Covid rules in No 10 and led to fresh claims he misled Parliament.
Under party rules, Mr Johnson will face a confidence vote in a secret ballot if 54 Tory MPs submit a letter to the chairman of the backbench 1922 Committee, Sir Graham Brady, calling for one.
Allies of the Prime Minister, including Home Secretary Priti Patel, have been urging the rebels to back off saying the country would not thank them for turning in on themselves at a time when people are struggling with a cost-of-living crisis.
However, some observers at Westminster have predicted that a challenge could come as early as next week, with the former party leader, Lord Hague of Richmond, warning that was Mr Johnson was in "real trouble".
Even if the 54 tally is not reached in the coming week, the Tories are facing a pair of difficult by-elections in Wakefield and Tiverton and Honiton later in the month which could be the trigger for a new tranche of letters if they are defeated.
