David Cameron has invoked the risk of war in a warning about the dangers of leaving the European Union.
The Prime Minister said peace in Europe could not be guaranteed and it is in the UK's interest to be able to influence what happens to our neighbours.
Mr Cameron's key speech marked an upturn in campaigning, and will be followed by an address from Boris Johnson, who is expected to challenge the Prime Minister's claims.
Introduced by former Labour Foreign Secretary David Miliband, Mr Cameron said: "For good or ill, we have written Europe's history just as Europe has helped to write ours.
"The moments of which we are rightly most proud in our national story include pivotal moments in European history. Blenheim. Trafalgar. Waterloo.
"Our country's heroism in the Great War. And most of all, our lone stand in 1940.
"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. 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. 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."
He said: "The serried rows of white headstones in lovingly tended Commonwealth war cemeteries stand as silent testament to the price this country has 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? I would never be so rash as to make that assumption."
Ahead of his speech, the Britain Stronger In Europe campaign group released a video featuring World War Two veterans calling on people to vote 'remain'.
Mr Cameron used WW2 leader Winston Churchill to boost his argument - the wartime PM has also been used by the Leave campaign.
A Vote Leave spokesman, responding to Mr Cameron's comments, said: "Claims that leaving the EU and taking back control would somehow lead to war smack of desperation from a campaign failing to make the case for the EU and our continued payment of £350m to Brussels every week.
"The PM's words are deeply ironic given the EU's own border agency says the EU's borderless policy is making the whole of Europe less safe. The safe option is to vote Leave."
