Sir John Major has warned that attempts to excuse the breaking of lockdown laws by No 10 is undermining trust in government and politics.

The former prime minister said the “foolish behaviour” and “evasive” tactics of ministers are having a “corrosive” effect on the UK’s democracy.

In a hard-hitting speech to the Institute for Government in London, he strongly condemned the way Boris Johnson has responded to the disclosures about lockdown parties in No 10.

“At No 10, the Prime Minister and officials broke lockdown laws,” he said.

“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.

“Collectively, this has made the Government look distinctly shifty, which has consequences that go far beyond political unpopularity. No Government can function properly if its every word is treated with suspicion.”

Sir John, a longstanding critic of Mr Johnson, said such conduct reflects a wider decline in standards in recent years.

“Trust in politics is at a low ebb, eroded by foolish behaviour, leaving a sense of unease about how our politics is being conducted. Too often, ministers have been evasive, and the truth has been optional.

“When ministers respond to legitimate questions with pre-prepared soundbites, or half-truths, or misdirection, or wild exaggeration, then respect for government and politics dies a little more.

“Misleading replies to questions invite disillusion. Outright lies breed contempt.

“In our democracy, we are able to speak truth to power. But if democracy is to be respected, power must also speak truth to the people. And yet, in recent years, they have not been doing so.”

Sir John said that, while the vast majority of elected politicians do not knowingly mislead, the behaviour of those who do is tarnishing the reputation of politics and of Parliament.

“If lies become commonplace, truth ceases to exist. What and who, then, can we believe? The risk is nothing and no-one. And where are we then?”

He said all this is taking place against the backdrop of the Prime Minister being investigated for several apparent breaches of the ministerial code.

“He chose to ignore critical reports on his ministers; rejected advice from his independent adviser on ministerial standards – who resigned; and attempted – but failed – to overturn a unanimous Standards Select Committee Report that condemned the behaviour of a parliamentary colleague and friend,” he said.

“It may be possible to find excuses for each of these lapses – and others – but all of them, taken together, tell a different tale.

“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.”