At the news conference, Obama defended his right to offer his opinion.
"In democracies, everybody should want more information than less. You shouldn't be afraid to hear an argument being made. That's not a threat. That should enhance the debate," he said.
In op-ed published Friday in the Daily Telegraph, Obama said that "a strong Europe is not a threat to Britain's global leadership; it enhances Britain's global leadership."
"Let me be clear, ultimately this is something the British voters have to decide for themselves," Obama told reporters at the news conference. "But as part of our special relationship, part of being friends is to be honest and to let you know what I think. Speaking honestly the outcome of that decision is a matter of deep interest to the United States because it affects our prospects as well."
— The Associated Press contributed to this report.