No 10 defends as 'serious and sober' PM's warning that Brexit would increase risk of war in Europe

Stressing how Mr Cameron made no apology for setting out the dangers of Brexit, he said: "It was a sober and principled speech. He is right to set out the risks and he is doing that."

Asked, given the risks of Brexit, if Whitehall plans were being put in place to avert a potential future war if Britain left the EU, the spokesman said: "The government has a position, which is, that we should vote to remain. The Prime Minister is setting out why that is the case and the risks of leaving. We are not doing any contingency planning for the referendum being a vote to leave."

Boris Johnson, the former London mayor and a leading light in the Vote to Leave campaign, said Mr Cameron’s warning of a threat to European peace and stability from Brexit was “very curious”.

"I don't believe that leaving the EU would cause World War III to break out on the European continent," he insisted.

In an address in Brussels today, Alex Salmond, the former First Minister, is set to use a speech to argue against relying on negative campaigning.

He will say: "With support for the EU across the UK hanging in the balance, the Prime Minister should reboot the Remain campaign away from Project Fear Mk 2. There is an overwhelming need in the UK to reset the debate and focus on a positive campaign to highlight the great achievements of the European ideal."

Following his speech at the British Museum in London, the PM dismissed suggestions he was “crying wolf” over the dangers of a future war in Europe after Leave campaigners accused him of desperate tactics.

Asked if there was a danger voters would think he was "crying wolf", the PM replied: "I would just say look at the speech, look at what I've said; consider the arguments. No-one can doubt that Europe has had a violent and turbulent history.

"These are facts. I am not arguing that the EU alone has kept the peace in Europe these last 70 years because, of course, Nato played a key role."

In his keynote address, the PM set out what he called the “big, bold patriotic case” for Britain to remain a member of the EU.

He said there were four reasons why membership made the UK better off, safer and stronger.

“First, what happens in Europe affects us, whether we like it or not, so we must be strong in Europe if we want to be strong at home and in the world.

“Second, the dangerous international situation facing Britain today, means that the closest possible co-operation with our European neighbours isn’t an optional extra, it is essential. We need to stand united. Now is a time for strength in numbers.

“Third, keeping our people safe from modern terrorist networks like Daesh and from serious crime that increasingly crosses borders means that we simply have to develop much closer means of security co-operation between countries within Europe. Britain needs to be fully engaged with that.

“Fourth, far from Britain’s influence in the world being undermined by our membership of the EU, it amplifies our power, like our membership of the UN or of NATO. It helps us achieve the things we want – whether it is fighting Ebola in Africa, tackling climate change, taking on the people smugglers. That’s not just our view; it’s the view of our friends and allies, too.”

Mr Cameron used historical references from Blenheim and Waterloo to World War One and the Battle of Britain to argue that for the UK to be a strong power in the world, it needed to be a strong power in Europe.

“Isolationism has never served this country well. 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 warned.

The PM said the serried rows of white headstones in lovingly-tended Commonwealth war cemeteries stood as silent testament to the price that Britain had paid to help restore peace and order in Europe.

“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?” he asked.

Mr Cameron pointed out that it was barely 20 years since war broke out in the Balkans and genocide occurred on the continent in Srebrenica.

“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 once 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. The truth is this: what happens in our neighbourhood matters to Britain. That was true in 1914, in 1940 and in 1989. Or, you could add 1588, 1704 and 1815. And it is just as true in 2016.

“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,” the PM declared.

Warnings by two former spy chiefs - Lord Evans of Weardale, former director-general of MI5, and ex-MI6 chief Sir John Sawers - that Brexit could harm the country's ability to fight terrorism were "unmistakable", Mr Cameron insisted.

He pointed out how the Paris and Brussels attacks were a reminder that "we face this threat together and will only succeed in overcoming it by working much more closely together".

Mr Cameron insisted border controls were not enough in the fight against terror. "It takes a network to defeat a network and European measures are a key weapon," he added.

The PM was welcomed on stage by Labour's former foreign secretary David Miliband, who said it was right for "conventional political divides to be put aside in the national interest" in the EU debate.

Labour leader Jeremy Corbyn has said that he will not share a platform with the PM during the referendum campaign but Mr Miliband said: "One of the things people dislike about politics is when people refuse to work together even though they agree. So now is the time to come together, to spell out the facts, and settle the argument."

He added: "I believe profoundly and to my core that Britain risks a gratuitous and dangerous act of political self-harm on June 23.

"Withdrawal from the European Union would damage this country, weaken our allies and embolden our enemies.

Meantime, in a keynote speech ahead of a UK tour, Mr Johnson said Britain would be "mad" not to take the once-in-a-lifetime chance of voting to leave the EE.

Speaking just hours after David Cameron warned that Brexit would be a threat to peace and stability in Europe, Mr Johnson insisted that it was Nato that underpinned peace while the EU was "a force for instability and alienation".

Comparing the EU to the Italian mafia, he warned of a campaign of "subterfuge" to hide from the British public constitutional changes, which, he said, were being introduced with the aim of creating "a country called Europe".

The ex-mayor rejected accusations that the Brexit camp was full of "little Englanders", arguing that withdrawal from the EU would allow the UK to trade and co-operate with its continental neighbours in a spirit of "liberal cosmopolitanism".

The Uxbridge MP - who has become the leading figurehead of the Leave campaign - said that the Remain camp relied on the three "wholly bogus" myths that EU membership boosted the economy and helped preserve peace and that a desire to quit the 28-nation union was anti-European.

"I am a child of Europe. I am a liberal cosmopolitan and my family is a genetic UN peacekeeping force," said Mr Johnson, who broke briefly into a German-language rendition of Beethoven's Ode To Joy to prove his point.

In Brussels, Mr Salmond, speaking on Europe Day, Mr Salmond will call on the UK to use its EU Council Presidency next year as an opportunity to influence the political process in Europe and set an agenda to boost jobs. He will also demand that a Scottish Minister is made chair of the EU Fisheries Council during this presidency.

Mr Salmond has been deeply critical of the Project Fear campaigns to date, describing them as "at best puerile and at worst outlandish scaremongering”.

He will insist that Scotland should secure its position in Europe in all circumstances:

"Let there be no mistaking that whatever the outcome, Scotland will have its voice heard. My hope and purpose over the coming six weeks is to campaign for a resounding vote for the UK to remain in the EU.

"However, as some polls suggest, should the result fall in favour of an English and therefore UK vote to leave but Scotland votes to remain, then the democratic decision becomes a constitutional conundrum.”

He said that was why First Minister Nicola Sturgeon had been clear that such a material change would bring about a “groundswell of anger loud enough to trigger a second Scottish independence referendum. Politicians and parties seeking to deny that right would be swept away by the political winds that would then be blowing”.

The Gordon MP said there were just 45 days left to take a positive pro-EU message to the voters.

"We want a Europe that prioritises growth and recovery, a Europe which reclaims the concept of a social contact and a Europe that responds with solidarity and humanity when faced with a refugee crisis.

"Europe must lead – the way it lifted itself 66 years ago today and resolved to work together for the better of all – to build a universal right and hope for every European citizen so that we can live together in peace and harmony.

“Only the united resolve of its 28 Member States will see this achieved in full and the UK must endeavour to turn its campaign for ‘remain’ into a force of hope and achievement," Mr Salmond will add.