U.S. President Barack Obama appealed directly to British voters on Friday to remain in the European Union, saying membership had magnified Britain's place in the world and made the bloc stronger and more outward looking.

In an article in the conservative Daily Telegraph -- which the eurosceptic newspaper placed on page 20 -- Obama invoked the interlinked history of the United States and Britain and the tens of thousands of Americans lying in European war graves as his reason for speaking on a June 23 referendum.

"The European Union doesn't moderate British influence - it magnifies it," he wrote under the headline: "As your friend, I tell you that the EU makes Britain even greater."

Obama, who opinion polls show is popular in Britain, arrived in London on Thursday to applaud Britain's EU membership which he said had helped make the world freer, richer and better able to tackle everything from migration to terrorism.

His remarks, which led television news broadcasts in Britain, undercut one of the most passionate arguments of the opponents of EU membership: that Britain could prosper on an equal basis with global powers such as the United States.

Obama said Britain's closest ally wanted it to remain in the club it joined in 1973 to bolster trade and strengthen the 28-member bloc, which Washington views as a pillar of stability.

"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."

A spokeswoman for Cameron welcomed Obama's intervention.

"When you see allies and partners coming out around the world advocating the case for the UK remaining in the EU, talking about how that amplifies our influence in the world, we think those are important arguments to hear," a spokeswoman for Cameron said.

"Stick Together"