President Barack Obama advocated for Britain to remain in the European Union while visiting London on Thursday. In a Daily Telegraph op-ed, the president wrote Britain’s ability to fight terrorism would be “more effective” if the nation remains with its European allies. “The European Union doesn’t moderate British influence—it magnifies it,” Obama wrote. However, Obama wrote it was ultimately a matter for the British voters to decide, but the “outcome of your decision is a matter of deep interest to the United States.” The president’s appeal comes as the U.K. continues to debate over a British exit from the EU in the upcoming June 23 referendum. Obama is slated to have lunch at Windsor Castle on Friday with Queen Elizabeth, who celebrated her 90th birthday Thursday, and her husband Prince Philip. He will then meet with Prime Minister David Cameron.