U.S. Republican presidential frontrunner Donald Trump said he thought Britain would vote to leave the European Union in a June 23 referendum because of concerns about high levels of migration.
"With the craziness that is going on with the migration, with people pouring in all over the place, I think that Britain will end up separating from the EU, that's my opinion," Trump said in an interview with ITV television broadcast on Thursday.
Opinion polls suggest Britain is divided on membership, with around a fifth of voters still undecided, and that migration is voters' top concern.
Trump, who has shocked some voters in Europe with proposals to build a wall along the border with Mexico and ban Muslims from entering the United States, said he was not endorsing any position in Britain's referendum.
But his comments contrast sharply with the public position of President Barack Obama, who has said Britain must remain in the 28-member bloc to maintain its global influence. Obama is due to visit Britain next month.
Britain's allies have said that an exit could weaken the West, which is grappling with the challenge of Islamist militants in Syria and Iraq and what Prime Minister David Cameron says is Russian President Vladimir Putin's increased aggression.
ITV showed the first excerpts of its interview with Trump on Wednesday, in which he said Muslims were not doing enough to report suspicious activity by extremists, comments that drew a rebuke from Britain's interior minister.
Britain has been divided over its place in Europe since France and West Germany sought closer unity to prevent a repeat of the destruction wrought by World War Two. Britain eventually joined the club, but remained a reluctant member, outside the core euro zone.
Opinion polls indicate that some British voters turned against membership during last year's European migrant crisis, even though very few of those streaming in from the Middle East and Africa end up in Britain. There are also indications that attacks such as those in Paris and Brussels increase support for a British exit.
Opponents of EU membership, such as UK Independence Party leader Nigel Farage, say Britain could prosper outside the club, and would be able to restrict migration more effectively. Pro-membership campaigners say such restrictions would cripple the economy.
Cameron promised in 2010 to reduce the annual level of net migration into Britain to below 100,000, but in the year to September 2015 it rose once again, to 323,000.
About half the migrants entering Britain are from other EU countries and can move around the bloc at will under its principle of freedom of movement.
Richard Dearlove, former head of the MI6 foreign intelligence service, said Britain could be safer if it voted to leave the EU because it would have greater control over immigration.
The comments, just days after the attacks in Brussels, contradicted Cameron's argument that, in an increasingly unstable world, Britain would be weaker and more insecure if it dropped out of the EU.
"The truth about Brexit from a national security perspective is that the cost to Britain would be low," Dearlove, head of the Secret Intelligence Service from 1999 to 2004, wrote in Prospect magazine.
"Europe would be the potential losers in national security," he said. "But if Brexit happened, the UK would almost certainly show the magnanimity not to make its European partners pay the cost."