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.
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.”