THE Tories’ internal strife on Europe has struck new heights with David Cameron’s warning that Brexit could increase the risk of war branded “desperate stuff” by the Leave campaign while the Remain camp denounced Boris Johnson as a “Putin apologist” for blaming the EU for the Ukraine crisis.

The ratcheting-up of the rhetoric came as Alex Salmond, in a speech in Brussels to mark Europe Day, urged both sides to drop Project Fear and reset the debate, saying the standard of discourse so far had been “at best puerile and at worst just outlandish scaremongering”.

In a keynote speech at the British Museum in London, the Prime Minister set out what he described as a “big, bold patriotic case” for Britain to remain a member of the EU.

Citing historical references from Waterloo to the Battle of Britain, he stressed how the UK could not isolate itself from the continent. “Whenever we turn our back on Europe, sooner or later we come to regret it. We have always had to go back in and always at a much higher cost,” he said.

But it was his reference to the consequences of Brexit on Europe’s peace and stability that caused most controversy.

“Britain has a fundamental national interest in maintaining common purpose in Europe to avoid future conflict between European countries. And that requires British leadership and for Britain to remain a member…”

Mr Cameron denied “crying wolf” while No 10 defended him, saying his speech had been a “serious and sober assessment” of the risks facing Britain if it ever left the EU.

But the Leave camp responded with a mixture of anger and disbelief. One senior Outer dismissed the PM’s remarks as “lunatic” and predicted they would backfire on the Remain campaign.

Former London mayor Boris Johnson, the leading Brexiteer, described the EU as "a force for instability and alienation" and dismissed his Tory colleague’s assertion, describing it as “very curious”. The London MP said: "I don't believe that leaving the EU would cause World War III to break out on the European continent.”

Leave.EU co-founder Arron Banks denounced Mr Cameron's comments as "frankly insulting" and added: “This is desperate stuff from Desperate Dave.”

Also in his speech, Mr Cameron, in the strongest example yet of playing the Union card, warned that Brexit could lead to the break-up of Britain.

"Let me just say this about Scotland; you don't renew your country by taking a decision that could ultimately lead to its disintegration. So, as we weigh up this decision, let us do so with our eyes open," declared the PM.

Meantime, in his keynote speech at Westminster ahead of a UK Vote Leave tour, Mr Johnson said Britain would be “mad” to forgo the once-in-a-lifetime chance of leaving the EU.

He claimed withdrawal from the EU would allow the country to trade and co-operate with its continental neighbours in a spirit of "liberal cosmopolitanism".

But he sparked controversy when he claimed: “If you want an example of EU foreign policymaking on the hoof and the EU’s pretensions to running a defence policy that have caused real trouble, then look at what has happened in Ukraine.”

Jack Straw, the former Labour Foreign Secretary, responded by saying: “Boris Johnson has plumbed new depths today by joining the likes of Farage, Le Pen and Wilders in blaming the EU rather than Vladimir Putin for what has happened in Ukraine.”

Today, Labour launches its short campaign to the June 23 poll, north and south of the border.

In London, party leader Jeremy Corbyn will join Labour In’s chief Alan Johnson to unveil the party’s new campaign bus while in Edinburgh this morning Frank Roy, the campaign director of Scotland Stronger in Europe, will launch the Scots campaign with a photo-call later in Glasgow.

In a separate development, the TUC this morning published analysis it claimed showed that around one million workers were at high risk of being forced to work excessive hours if the UK left the EU.

Elsewhere, a poll showed support for Britain staying in the EU among business leaders was hardening with the Institute of Directors’ snapshot of 1,224 members finding 63 per cent backed Remain, up from 60 per cent in February, while 29 per cent supported Leave, down from 31 per cent.