Boris Johnson and his officials "broke lockdown laws" over parties held in Downing Street, Conservative former Prime Minister Sir John Major has said.
He accused the government of feeling it "need not obey the rules", adding: "Outright lies breed contempt."
Sir John's comments come while 12 gatherings are still being investigated by the Metropolitan Police.
Mr Johnson, who is coming under pressure to quit, said he was looking forward to the process being completed.
That would be "the time to say something" on the matter, the PM added, as he answered questions during a press conference in Brussels.
The Met is reviewing its previous decision not to investigate a Christmas quiz in No 10 in 2020, after the
Mirror published a photograph of Mr Johnson with three aides - wearing tinsel and a Santa hat - near a bottle of sparkling wine.
It also announced on Wednesday that it was emailing more than 50 people as part of its existing inquiry into lockdown parties.
Mr Johnson - who is known to have attended three gatherings - and wife Carrie are among those expected to be contacted.
The Met said people emailed would not necessarily have to pay a fine, adding that this would "normally" happen where officers believed regulations had been breached without reasonable excuse.
But, in a speech to the Institute for Government think tank, Sir John said: "At No 10, the prime minister and officials broke lockdown laws.
"Brazen excuses were dreamed up. Day after day the public was asked to believe the unbelievable. Ministers were sent out to defend the indefensible - making themselves look gullible or foolish."
Sir John, in office from 1990 to 1997, added: "The prime minister and our present government not only challenge the law, but also seem to believe that they, and they alone, need not obey the rules, traditions, conventions - call them what you will - of public life.
"The charge that there is one law for the government, and one for everyone else is politically deadly - and it has struck home."
Sir John, who has previously criticised Mr Johnson over his handling of Brexit and
called the government "politically corrupt" in its treatment of the House of Commons, said: "When ministers respond to legitimate questions with pre-prepared sound bites, or half-truths, or misdirection, or wild exaggeration, then respect for government and politics dies a little more."
He urged the prime minister to introduce a "fully independent" guardian of ethics in politics and new laws to limit political funding by individuals, companies and trade unions.
Labour, the Liberal Democrats, the SNP and some Conservative MPs have called for Mr Johnson to resign over the Downing Street gatherings.
If 54 Tory MPs write letters declaring no confidence in the prime minister, this will prompt a vote by all Tory MPs, which would see Mr Johnson ousted, should he lose.
The Met investigation began after Sue Gray, the senior civil servant looking into the gatherings, passed on her findings.
Her
interim report criticised "failures of leadership and judgement" within government, while her full findings are expected to come out after the Met concludes its investigation.
Mr Johnson said; "That [Met investigation] process must be completed and I'm looking forward to it being completed, and that's the time to say something on that."
And Defence Secretary Ben Wallace told BBC Radio 4's Today programme: "The prime minister has said he will be full and transparent. He will notify [the public] if he is receiving any form of fine et cetera."
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