Boris Johnson has warned voters that choosing a “Corbyn-Sturgeon technicolour coalition of chaos” would leave Britain facing a nightmare on Downing St come December 13.

In his first keynote speech of the General Election campaign, the Prime Minister insisted voting Conservative was the "only way" to get Brexit done after Nigel Farage refused to bow to pressure to pull his party's candidates from Tory/Labour marginal seats.

Addressing workers at an electric vehicle factory in Coventry, which makes taxi-cabs, the Conservative leader unveiled plans to ramp up research and development funding alongside a series of green energy spending pledges designed to woo voters.

Mr Johnson said he wanted to double taxpayer-funded research and development contributions to £18 billion by 2024/25, with an intention to help advanced maths research, nuclear fusion, health work and a national space strategy.

His announcement came as a Savanta Comres poll gave the Tories a 10-point lead over Labour, 40 per cent to 30, with the Liberal Democrats on 17 and the Brexit Party on seven. The snapshot was conducted after Mr Farage said he would stand down his party’s candidates in more than 300 Tory-held seats.

In his speech, Mr Johnson complained how Brexit had been "paralysing politics" for three and a half years and insisted only a vote for the Tories would get Brexit done.

"Because a vote, alas, for any other party would simply lead to a premiership of Jeremy Corbyn in coalition with Nicola Sturgeon and…two referendums next year, when the people of this country really want to make so much more progress on other things."

Mr Johnson expressed some bafflement at the Labour leader’s earlier remarks during his two-day campaign visit to Scotland about whether a future Corbyn Government would facilitate a second Scottish independence referendum, saying they were “veiled in mystery”.

But, nonetheless, he insisted the SNP leader was Mr Corbyn’s “path to power,” explaining: “It is hard to see how he could very easily turn her down if that[a second referendum]were to be the condition that she would put on a coalition as she surely would.”

He warned his audience: “You don’t want to wake up on Friday the 13th and find a nightmare on Downing St and a Corbyn-Sturgeon technicolour coalition of chaos.”

Earlier in Scotland, Mr Corbyn caused a deal of confusion about his position on indyref2. At first he told reporters there would be "no referendum in the first term for a Labour Government" ie five years even if the SNP won a majority of Scottish seats at the election but then insisted he would "not countenance" another independence referendum in "the early years" of a Labour Government.

When quizzed later by journalists, he told them: “I think the confusion is with you, if I may say so, not me.”

Later, Alister Jack, the Scottish Secretary, mocked the Labour leader, saying: “Jeremy Corbyn’s position on Scottish independence is as incoherent as his position on Brexit. In the space of a few hours he has ruled out a second Scottish independence referendum and then ruled out ruling it out.”

In a question and answer session following his speech in Coventry, Mr Johnson, who had earlier received a frosty reception from flood victims when he visited South Yorkshire, declined to apologise when challenged over why voters in the flood-affected areas should vote for him.

The PM insisted UK ministers stood “ready to support in any way” the 850 residents whose homes had been flooded.

He pointed out the Government had already put some £2.6 billion into flood defences and would do more. It was made clear that up to 400 troops would be available to be deployed if necessary.

Meanwhile, the Tory leader criticised Mr Corbyn’s comments about IS leader Abu Bakr al-Baghdadi after he was killed during a raid by US special forces last month.

The Labour leader had earlier told LBC radio: "If we preach international law and international legal process through the International Court of Justice in the Hague, then we should carry it out and if it's possible to arrest somebody and put them on trial, then that is what should have been done.

"That is what I said about the death[of Osama bin Laden] in 2011 and it will continue to be my principle.”

But Mr Johnson, describing al-Baghdadi as “an absolute diabolical foe of this country, of our liberal values, everything we believe in and support,” denounced Mr Corbyn’s comments as “naive to the point of being dangerous”.

Al-Baghdadi blew himself up with a suicide vest after a strike by US special forces on his hideout in Syria.