US president Barack Obama has delivered an impassioned defence of the European Union, warning that America and the world need a "united" Europe.
In comments which are likely be seen as a further plea for a Remain vote in Britain's June 23 referendum, Mr Obama hailed the EU as "one of the greatest political and economic achievements of modern times" and cautioned individual states against rebuilding barriers which existed in the 20th century.
Mr Obama's intervention, in a speech in Germany, came as Brexit's biggest hitters sought to seize back the referendum initiative by putting immigration at the top of the agenda.
Justice Secretary Michael Gove warned the UK faces a migration "free for all" unless it breaks away from Brussels as the Leave camp moved to exploit an admission from the Government that EU free movement of labour rules make it harder to curb immigration.
But the US president warned of the dangers of an "increasing intolerance" in politics which promoted an "us versus them" mentality towards migrants.
Speaking at the start of the G5 summit in Hanover, where he will discuss security threats with David Cameron, Chancellor Angela Merkel, French president Francois Hollande and Italian PM Matteo Renzi, Mr Obama acknowledged that anxieties over globalisation, terrorism and immigration were "real and legitimate".
"All these challenges have led some to question whether European integration can long endure, whether you might be better off separating off, redrawing some of the barriers and the walls between nations that existed in the 20th century," said the US president
But he added: "If a unified, peaceful, liberal, pluralistic, free-market Europe begins to doubt itself, begins to question the progress that's been made over the last several decades, then we can't expect the progress that is just now taking hold in many places around the world will continue.
"Instead, we will be empowering those who argue that democracy can't work, that intolerance and tribalism and organising ourselves along ethnic lines and authoritarianism and restrictions on the press - that those are the things that the challenges of today demand.
"I've come here today to the heart of Europe to say that the United States and the entire world needs a strong and prosperous and united Europe."
President Obama said he understood that dealing with Brussels could be "frustrating" and slow down decision-making.
But he said the union had brought peace among its members and insisted that a strong, united Europe was vital for global security and prosperity.
He said: "European unity can require frustrating compromise. It adds layers of government that can slow decision-making.
"I understand. I have been in meetings with the European Commission.
"And, as an American, we are famously disdainful of government. We understand how easy it must be to vent at Brussels and complain.
"But, remember that every member of your union is a democracy. That's not an accident.
"Remember that no EU country has raised arms against another. That's not an accident."
President Obama said a "strong united" Europe remains a "necessity for all of us".
"It's a necessity for the United States because Europe's security and prosperity is inherently indivisible from our own," he added.
"A strong united Europe is a necessity for the world because an integrated Europe remains vital to our international order."
President Obama said Islamic State was the "most urgent threat" to Western nations and warned the EU it "could do more" through air strikes, military trainers and economic assistance to stabilise Iraq.
Urging all EU countries to meet the Nato target of spending 2% of national income on defence, he added: "Sometimes Europe has been complacent its own defence."