US President Barack Obama made an appeal for Britain to continue to stay in the European Union, saying membership had increased Britain's place in the world.
Fearing that a British exit could weaken West, the US president arrived in London to tell Britons that issues of migration, terrorism and economic slowdowns could be tackled more successfully with the UK in the EU.
"The European Union doesn't moderate British influence - it magnifies it," he wrote in the Daily Telegraph newspaper.
"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".
Obama's visit to Britain s a welcome one for Cameron, who is leading "In" campaign, but it has drawn criticism from those stressing Britain should leave the bloc.
US government, and many US banks, fear that Britain’s exit would cause market turmoil, reduce the clout of its strongest European ally, undermine London's global financial hub status, cripple the EU and weaken Western security.
"Now is a time for friends and allies to stick together," he said, adding that ultimately the referendum was a matter for British voters to decide.