Boris Johnson is a serial liar, who could be facing prosecution for “misconduct in public office” if UK judges next week rule that he misled the Queen over the suspension of Westminster, Ed Davey, the deputy leader of the Liberal Democrats, has suggested.

His stark comments, in an exclusive interview with The Herald ahead of the start of his party’s annual conference in Bournemouth this weekend, came as the Prime Minister – heckled by a protester while on a government visit to Yorkshire - faced a barrage of personal attacks.

In his long-awaited memoirs, David Cameron lashed out at his erstwhile friend and Government colleague, saying Mr Johnson had left the “truth at home” over Brexit and accused him of “sharp practices” in his deeply contentious move to prorogue the UK Parliament. The ex-Tory leader also branded Cabinet Office Minister Michael Gove, another leading Leaver, “mendacious” and even referred to him as a “w****r”.

And last night at the Business for Scotland annual dinner in Glasgow, Nicola Sturgeon insisted Scotland deserved better than a “reckless” UK Government led by a duplicitous premier.

“Boris Johnson, who twice lost his job for lying and posed beside that notorious Brexit bus, is now the Prime Minister, who appears to have misled the Queen, misled Parliament and misled the country,” declared the First Minister.

Sitting in his London home before heading to the Dorset coast, Sir Ed claimed he was deeply worried about the threat to the 300-year-old Union now posed by Mr Johnson, whom he described as an “English Nationalist populist”.

The Herald: Camley's Cartoon: Johnson under fire.Camley's Cartoon: Johnson under fire.

He explained he had a “huge worry” that on the Right, where the PM and Nigel Farage, “vehemently for a hard deal or a no-deal Brexit…giving grist to the mill of the Scottish Nationalists,” and on the Left was a “deeply left-wing and pro-Brexit” Jeremy Corbyn, ready to do a deal with the SNP.

“So, you have these two coalitions on the hard Right and the hard Left, who are putting Britain’s future in the EU and the United Kingdom at risk.

“One of our core messages, of course, is: stop Brexit; but it should also be: save the UK…We are the only party who can save the UK. If you want to vote for a Unionist party in Scotland to stop the Nats, we are now the only choice. The Conservatives have completely left the field.”

The former Energy and Climate Change Secretary referenced Ruth Davidson, who two weeks ago unexpectedly quit as the Scottish Conservative leader, saying: “We know where she is coming from politically; I don’t want to put words in her mouth but she thinks Johnson is the biggest threat to the Union the Tories have ever produced. This is deeply worrying.”

Sir Ed then launched into a withering attack on the PM’s honesty, saying: “The problem with Mr Johnson is that because he lies so often, he almost lies to himself. He doesn’t recognise the truth anymore, which is a shocking thing in a prime minister. So, his level of self-deception is enormous…

“No one trusts him. This is a prime minister who was sacked by his newspaper editor for making up stories, sacked by his party leader for lying…because he lied about an affair after lying to his wife…He lies in political campaigns, he lies at the dispatch box, he may well have lied to the Queen. There is a common theme here; it’s deeply unpleasant,” insisted the Kingston and Surbiton MP.

Asked if the PM could survive in office if the UK Supreme Court’s 11 judges next week agreed with the justices at Scotland’s Court of Session - that the PM’s decision to prorogue Parliament was unlawful “because it had the purpose of stymying Parliament” – the Lib Dem deputy leader said: “No.”

When it was suggested that Mr Johnson might even face criminal prosecution for misleading the head of state, he interjected: “Misconduct in public office. I have heard a lot of thought on impeachment.”

Misconduct in public office is an ancient common law offence, which carries a maximum sentence of life imprisonment.

Sir Ed went on: “The country is in a chaotic mess; there is still a threat of no-deal, we have a political crisis, a prime minister out of control. He and Dominic Cummings[his chief aide] are creating chaos…Those of us, who want to get rid of Brexit, have to show we are going to do things in a calm, considered and responsible way.

“If we do that and show we are the ones who can restore things, save the UK, end this Brexit chaos, protect our democracy and the things we care about, people will want to hear that,” he added.

While on a visit to meet civic and business leaders in Rotherham, the PM’s address was interrupted by a lone heckler, who told him: “Get back to Parliament…Why are you not with them in Parliament sorting out the mess that you have created?”

Momentarily flummoxed, Mr Johnson insisted there would be "ample time" for MPs to debate any Brexit deal and he would take Britain out of the EU on October 31 “whatever the shenanigans that may be going on at Westminster”.

This appeared to be a thinly-veiled swipe at John Bercow, the Commons Speaker, who this week made clear MPs were ready to use "procedural creativity" to block the PM if he tried to force through a no-deal Brexit in breach of the law to extend Brexit to January 31.

Ahead of a working lunch on Monday in Luxembourg with Jean-Claude Juncker, the outgoing President of the European Commission, and Michel Barnier, the EU’s chief Brexit negotiator, Mr Johnson expressed himself to be “cautiously optimistic” about securing a new deal before the October 31 deadline.

However, a senior No 10 source played down the prospect of an imminent breakthrough, saying: “The PM is working hard to get a deal and we have been putting forward ideas but there is a long way to go."

Meanwhile, Arlene Foster, the DUP leader, dismissed reports her party was about to offer the UK Government a lifeline to help it unlock a Brexit deal by accepting an all-Ireland food zone to help scrap the backstop.

"The UK must leave as one nation,” she declared, tweeting: “We are keen to see a sensible deal but not one that divides the internal market of the UK. We will not support any arrangements that create a barrier to East West trade."

In Dublin, Leo Varadkar, the Taoiseach, made clear that what the UK had thus far offered to solve the backstop “falls very far short of what we need”.