Dire warning: David Cameron will deliver a dramatic speech that will invoke Winston Churchill, the Second World War and the graves of the fallen

Europe risks sliding back into conflict and genocide if Britain votes to leave the EU, David Cameron will say today.

In an extraordinary escalation of the referendum battle, he will invoke Winston Churchill, the Second World War and the graves of the fallen.

The Remain camp will also wheel out military veterans in an emotive video warning against jeopardising the sacrifices of the dead.

Out campaigners have accused Downing Street, which yesterday claimed house prices would collapse following a vote to leave, of desperation. They say No 10 is panicking with the polls neck and neck despite the intervention of Barack Obama and a series of dire warnings about the risks of Brexit.

Historians have dismissed the suggestion that the EU had kept the peace in Europe, citing instead the crucial role of Nato.

But in a speech to mark the start of the final 45 days of the referendum contest the Prime Minister will insist a leave vote would be catastrophic.

'Isolationism has never served this country well,' he will say. '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 much higher cost.'

In other developments in the increasingly bitter referendum fight:

Downing Street was accused of trying to scupper TV debates;

The former head of MI5 was dragged into a row over whether British citizens are safer inside the EU;

Environment Secretary Liz Truss claimed 40,000 jobs in the Scottish whisky industry would be put at risk.

The Prime Minister's speech, described as his biggest of the campaign so far, will claim that 'the serried rows of white headstones in Commonwealth war cemeteries stand as silent testament to the price this country has paid to help restore peace and order in Europe'.

He will add: 'Can we be so sure that peace and stability on our continent are assured beyond any shadow of doubt?

'Is that a risk worth taking? I would never be so rash as to make that assumption. It's barely been 20 years since war in the Balkans and genocide in Srebrenica.

Lone stand: David Cameron will say that Britain stood as a bulwark against a new dark age of tyranny and oppression in 1940. Pictured, Hitler's SA officers on an exercise outside Munich

'In the last few years, we have seen tanks rolling into Georgia and Ukraine. And of this I am completely sure. The European Union has helped reconcile countries which were at each others' throats for decades.

'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 will also cite the threats to Britain from the Spanish Armada and in the Napoleonic Wars.

He will say: 'We know that to be a global power and a European power are not mutually exclusive. And the moments of which we are rightly most proud in our national story include pivotal moments in European history – Blenheim, Trafalgar, Waterloo.

Striking back: Boris Johnson will also speak today

'Our country's heroism in the Great War. And most of all our lone stand in 1940, when Britain stood as a bulwark against a new dark age of tyranny and oppression. But it wasn't through choice that we were alone. Churchill never wanted that. He spent the months before the Battle of Britain began trying to keep our French allies in the war, and then after France fell, he spent the next 18 months persuading the United States to come to our aid.

'And in the post-war period he argued passionately for Western Europe to come together, to promote free trade, and to build institutions which would endure so that our continent would never again see such bloodshed.

'The truth is this: what happens in our neighbourhood matters to Britain. Either we influence Europe, or it influences us.

'And if things go wrong in Europe, let's not pretend we can be immune from the consequences.'

Mr Cameron's intervention will take place at 8am, three hours before Boris Johnson, a biographer of Churchill, is due to deliver a major speech for the Leave campaign.

He is expected to strike back against the Prime Minister by seeking to 'debunk and destroy the myth that the EU single market has ever done anything useful'.

The former London mayor will then embark on a nationwide bus tour starting on Wednesday.

Leading historians, including the former head of history at Cambridge University, Professor David Abulafia, have dismissed the claim that the EU has brought peace to Europe as 'historically illiterate'.

They say it is Nato that has kept us safe since 1945. It was also the Nato alliance that managed Europe's defence against the Soviet Union in the Cold War and which organised European action after 9/11, it is argued.

But to ram home the PM's point, the Britain Stronger in Europe campaign will release a video in which four war heroes will make the 'patriotic case' for the EU.

Harry Leslie Smith, David Meyland, Patrick Churchill and Field Marshal Lord Bramall will all make their arguments for staying in the EU over the 'isolationism' that would be foisted on Britain.