LONDON -- Lending political backup to a struggling friend, President Barack Obama made an impassioned plea to Britons to heed Prime Minister David Cameron's call to stay in the European Union and dismissed critics who accused the U.S. president of meddling in British affairs.

Speaking at a press conference at 10 Downing Street, Obama told reporters that Britain's power is amplified by its membership in the 28-nation union, not diminished. He made an almost sentimental appeal to the "special relationship" between the two countries. And he said cast a grim picture of the economic stakes --saying flatly the U.S. would not rush to write a free trade deal with a newly independent Great Britain.

"Let me be clear, ultimately this is something that the British voters have to decide for themselves but ... part of being friends is to be honest and to let you know what I think," he said. "It affects our prospect as well. The United States wants a strong United Kingdom as a partner."

Obama spoke on the first day of a three-day visit to London, likely the last of this presidency. The visit comes two months before a June referendum on leaving the union. Polls suggest it will be a close-fought race, with most phone polls indicating a lead for the Remain campaign while some online polls put the Leave camp ahead.

Obama described the votes as potentially damaging to the British economy. He said the U.S. is focused on writing a massive trade agreement with the European Union and would not prioritize a bilateral agreement with the UK. Britain would have to get "in the back of the queue," he said.

As he landed Thursday night, the president laid out his arguments in an op-ed in a London newspaper, harkening back to the "special relationship" forged by wartime allies President Franklin Roosevelt and Prime Minister Winston Churchill. With that special status comes with leeway to interfere, Obama argued, writing that he was offering his thoughts with the "candour of a friend."

Obama's candour wasn't universally appreciated. In increasingly heated language, critics accused Obama of meddling in British business. Former London Mayor Boris Johnson, the head of the Leave campaign called Obama's advice "paradoxical, inconsistent, incoherent" and suggested Obama's background played a role.

Writing in The Sun newspaper, Johnson recounted a claim that a bust of former British Prime Minister Winston Churchill was removed from the Oval Office after Obama was elected and returned to the British Embassy. The White House has said that the Churchill bust is still in a prominent place in the presidential residence.

Johnson wrote that some said removing the bust "was a symbol of the part-Kenyan president's ancestral dislike of the British Empire, of which Churchill had been such a fervent defender."

Obama's late father was from Kenya, a former British colony that gained independence in the 1960s.

Obama has remained a broadly popular figure in Britain, although reliable surveys are scarce. In June 2015, three-quarters of Britons told pollsters they had confidence in his judgment on world affairs, according to a Pew Research survey.

That goodwill hasn't kept Britons in breaking from U.S. at key moments. In 2013, as Obama leaned on Cameron to join in threatened airstrikes in Syria, the House of Commons rejected the idea.

There have been other recent signs of stress on the relationship. British officials bristle over Obama's recent comments in the Atlantic magazine, in which he said he regretted trusting Europeans to stabilize Libya after the 2011 death of strongman Moammar Gadhafi. He specifically said Cameron had become "distracted by a range of other things" while Libya devolved into chaos.