"The U.S. and the world need your outsized influence to continue -- including within Europe," Obama said.
Opponents of the EU, many of whom laud the U.S. alliance, have said that membership has shackled Britain to the corpse of a failed German-dominated experiment in European integration, and that Britain, if freed, could prosper as a sole trader.
Such opponents caution that while many world leaders like Obama, banks such as Goldman Sachs and big companies such Ford may want Britain to remain in the EU, the British people do not appreciate being lectured about how to vote.
Nigel Farage, a prominent opponent of membership as leader of Britain's UK Independence Party, called Obama the most anti-British American president to date.
"This is an unwelcome interference ... Mercifully, he won't be in office for much longer."