U.S. President Barack Obama will come to London in April and urge British voters to back continued membership of the European Union, The Independent on Sunday newspaper reported.
Britons vote in June on whether to remain part of the world's biggest trading bloc and Obama has previously said he wants Britain to stay in the EU and help maintain the post-war transatlantic partnership.
Most opinion polls show voters are split with a large number of people as yet undecided.
Obama is due to attend a technology fair in Germany in late April and a source told the Independent on Sunday he would visit the British capital around that time.
"It would be pretty shocking if he didn't ask voters to stay in the EU," the source was quoted as saying.
The White House declined to comment and Prime Minister David Cameron's office did not offer an immediate comment when contacted by Reuters.