US President Barack Obama on Friday urged British voters to stay in the European Union, telling them the United States "needs your influence to continue."
"The United States sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the EU open, outward looking, and closely linked to its allies on the other side of the Atlantic," Obama, who arrived in London late Thursday, wrote in the Telegraph newspaper.
"So the US and the world need your outsized influence to continue - including within Europe," he said.
Britons will go to the polls on June 23 for a referendum on whether to stay or leave the EU, with Prime Minister David Cameron leading efforts for the country to remain in the bloc.
Campaigners for a Brexit, or British exit from the EU, criticised Obama's intervention. Iain Duncan Smith, a leading eurosceptic in Cameron's Conservative Party, accused the US president of trying to "intimidate the British people into voting to stay in the EU."
Writing in The Sun, London mayor Boris Johnson, another Conservative backer of Vote Leave, called Obama's appeal to Britain "a breathtaking example of the principle of 'do as I say, but not as I do'."
But Cameron backed Obama's right to give his views on Brexit during his visit to Britain, which began late Thursday.
"The US is one of our closest allies," Cameron wrote on Twitter. "So it's important to hear Barack Obama on why we should remain in the EU."
Obama met US embassy staff in London early Friday before a scheduled lunch with Queen Elizabeth II at Windsor Castle, her residence outside London, and a meeting with Cameron later Friday.
Obama and Cameron are expected to discuss Libya, Syria, the fight against Islamic State and other international issues. They are scheduled to hold a joint press conference at Downing Street, the prime minister's official residence in London.
They are also expected to reaffirm the "special relationship" between the two nations, after the White House said ahead of Obama's visit that the United States has "no closer friend in the world" than Britain.
In The Telegraph, Obama stressed the EU's role in pursuing a nuclear deal with Iran, and the Paris climate agreement, and also addressed the key economic debate for and against Brexit.
"When it comes to creating jobs, trade, and economic growth in line with our values, the UK has benefited from its membership in the EU," he wrote.
He said that in the end, however, the vote was "a matter for British voters to decide for yourselves."
A controversy over Obama's reiteration of his opposition to a British exit from the EU, or Brexit had begun before his arrival.
An open letter from UK Independence Party leader Nigel Farage and other founders of the Leave movement asked Obama to "abstain from any intended advocacy" on the referendum, while an online petition to parliament, signed by some 35,000 people, called for him to be gagged.