Boris Johnson has warned voters against ushering a "technicolour" coalition into Number 10 - including Labour and the SNP - as he fired the starting gun on the general election campaign.

The Prime Minister visited the Queen to formally agree to dissolve parliament but his campaign got off to a rocky start after the resignation of Welsh secretary Alun Cairns.

The long-serving Conservative cabinet minister quit after being accused of "brazenly lying" about his prior knowledge of a collapsed rape trial involving a Tory candidate in Wales.

The PM's speech was delayed as a result of the controversy.

Speaking outside Number 10 following his audience with the Queen, Johnson said a Labour government with SNP support would result in "two referendums" in 2020 - one on Brexit and another on independence.

It comes as First Minister and SNP leader Nicola Sturgeon said the SNP winning the most seats in Scotland would make her calls for indyref2 "irresistible".

The Prime Minister said he did not want an early election but that MPs were refusing to deliver Brexit.

He said: "We've got to the stage where we have no choice because our parliament is paralysed, it's been stuck in a rut for three-and-a-half years.

"And I'm afraid our MPs are just refusing, time and again to deliver Brexit and honour the mandate of the people.

"I can tell you, I've got to the stage where I've been wanting to chew my own tie in frustration because, in a sense, we're so nearly there.

"We've got a deal, oven-ready, by which we can leave the EU in just a few weeks."

The Prime Minister said a "technicolour yawn of a coalition" involving Labour leader Jeremy Corbyn would spend 2020 having two referendums.

He continued: "One on Scotland because he's done a deal with the Scots nationalists to assist the breakup of the Union if they sustain him in power.

"And another referendum on Brexit, which is meant to happen in nine months' time after he's renegotiated supposedly our exit and renegotiated this deal.

"And what is his plan for that renegotiation? What question would be put to the public? We don't know. What are the options? We don't know."

On the campaign trail in Shropshire, the Labour leader was spelling out his tax plans - as Scottish Labour leader Richard Leonard launch his party's campaign in Glasgow.

Corbyn said: "My personal views on billionaires is that they've obviously got a great deal of money therefore they're in a very strong position to pay a lot more tax.

"So our tax plans will affect the richest 5% of our society.

"We will be chasing down tax evasion, tax avoidance and tax havens because at the end of the day if you're doing some very clever wheeze, which somehow or other is avoiding your levels of taxation, you should be paying.

"Go further away,what happens then? You've got an underfunded school, hospital and public services as a whole."

He added: "You've got a moral obligation to pay your taxes. However rich, powerful and famous you are, one day you might have a heart attack. Then you're going to need the public services."

Launching Scottish Labour's campaign on Wednesday, Leonard told voters in Glasgow's Maryhill: "Scotland has the power to decide if Boris Johnson or Jeremy Corbyn is in Downing Street by the end of the year."

The First Minister says the country's electoral maths show that only the SNP can "stop the Tories" in Scotland and "deprive them of a majority".

She has sent an open letter to Remain voters, urging them to back the SNP to help evict Johnson and his Conservative party from government.

And campaigning in Alloa on Wednesday, Sturgeon said: "It is my intention to have a (independence) referendum next year.

"On this question of will Westminster allow it or not, we are at the start of an election campaign - this is an opportunity for the people in Scotland to have their say and make their views known.

"If the SNP win this election, I think that demand becomes irresistible."

She suggested Labour is already "pretty much conceding" it could not stand in the way of a second independence vote, and added: "I suspect it won't be too much longer until we see the Tories struggle to maintain that argument as well."

The Scottish Liberal Democrats accused the SNP leader of trying to "hoodwink" Remain voters.

MSP Alex Cole-Hamilton said: "Nicola Sturgeon is trying to pull the wool over voter's eyes. Every vote for the SNP will be used to justify their independence obsession.

"It took months of pestering by the Liberal Democrats and others before Nicola Sturgeon reluctantly backed a People's Vote.

"The SNP would need to introduce brutal spending cuts before an independent Scotland could get back into the European Union.

"Remain voters should ignore the leaky lifeboat of independence and help prevent the ship from sinking by voting Liberal Democrat."