Prime Minister David Cameron warned Monday that a British exit from the EU would threaten peace on the continent, as the campaign for next month's crucial referendum gathered steam after regional elections.
With polls showing the two campaigns are neck-and-neck, Cameron and the de-facto leader of the "Leave" movement, former London mayor Boris Johnson, were both stepping up efforts to woo undecided voters.
Leaving the 28-member bloc in the June 23 referendum would be a "reckless and irresponsible" risk to Britain's economic stability that would leave it "permanently poorer", Cameron warned.
He also said a "Brexit" would threaten Britain's strength and security in the world, along with peace on the continent if "Europe's foremost military power" quit the European Union.
"Isolationism has never served this country well," he said in a speech at the British Museum in London.
"Whenever we turn our back on Europe, sooner or later we come to regret it. We've always had to go back in, and always at a much higher cost."
Cameron said that while Europe had largely been at peace since the end of World War II, it was barely two decades since the Bosnian war, while the continent was facing a "newly belligerent Russia", with conflicts in Georgia and Ukraine.
British war graves on the continent "stand as silent testament to the price this country has paid to help restore peace and order in Europe", he said.
"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."
Leave campaigners attacked the speech with Nigel Farage, leader of the anti-EU UK Independence Party, saying the EU had increased hostility on the continent, especially in Greece.
"The anti-democratic EU, far from bringing countries together, has divided Europe," he said.
"If we remain in the EU, we will be locked inside a failed project that is pushing towards closer fiscal, political and military union, no matter the pain and suffering it brings to the people of Europe."
The Remain and Leave camps are neck-and-neck on 50 percent each, according to the What UK Thinks website's average of the last six opinion polls.
Later Monday, Cameron's fellow Conservative Johnson was also making a speech in which he decried the EU as a "28-man pantomime horse".
The referendum campaign was picking up pace again after regional and local elections last Thursday which saw Labour's Sadiq Khan elected as London's new mayor, and pro-independence nationalists returned to power in Scotland, albeit without a majority.
Scottish First Minister Nicola Sturgeon has said there would be an "almost certain" drive for another referendum on Scottish independence if the UK voted to leave the EU but Scottish voters wanted to stay.
Cameron raised this prospect during his speech.
"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," he said.
Cameron's Conservative Party is deeply divided over the EU referendum with only around half of his MPs with him on the Remain side.
The prime minister insisted again on Monday that the party would be able to heal its rifts following the vote.