US President Barack Obama's comments on a possible Brexit during his visit to London in April have helped boost support for Britain leaving the European Union, according to a former adviser to late British prime minister Margaret Thatcher.

"Most British people fully rejected the idea that President Obama should be lecturing them on how they should be voting on their own referendum," said Nile Gardiner, director of the Margaret Thatcher Center for Freedom at the Heritage Foundation, a conservative think tank in Washington.

Gardiner was referring to Obama’s statement that if Britons vote to leave the EU, it would put the country at "the back of the queue" for achieving a bilateral free trade deal with the United States.

He said that it had been received in Britain as a "slap in the face." Many people believed the opposite: that Britain would be at the front of the queue for a free trade deal if it were out of the EU. And there would be "tremendous momentum" for that on Capitol Hill, he said, speaking Wednesday at an overwhelmingly pro-Brexit forum sponsored by the Heritage Foundation.

He and the other panellist agreed that the special relationship between the US and Britain would be strengthened through such a trade deal and other bilateral arrangements if Britain left the EU.​

Obama's "back-of-the-queue" comment was "an extraordinary intervention by a US president," Gardiner said, not just in the sense that he gave "the wrong advice," but that he spoke "in a condescending tone toward a very close friend and ally."

Gardiner and the other speakers agreed that Britain should exit the union to escape burdensome regulation and economic decline and to regain control of its borders.

Citing an influx of 270,000 people to Britain from the European Union in 2015, he said immigration was at a staggering level.

"I think the British people are opposed to the idea that they have no say over levels of immigration from Europe and there is a fundamental lack of democratic consent here with regard to immigration," Gardiner said.

Gardiner also said he wanted to set record straight on what his former boss's view of Brexit would have been because there had been some "mischievous" articles written suggesting Thatcher would have opposed it.

"Based on my own conversations with her, she would be fighting tooth and nail [for Brexit]," he said.

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