United States President Barack Obama will head to London next week to join the international chorus of leaders imploring Britain not to leave the European Union.

The White House said on Thursday that the president will arrive in Britain on April 21 for a three-day visit, just two months before the country votes in an referendum on the issue.

"He will make very clear that this is decision for the people of the United Kingdom to make," Obama's close foreign policy adviser Ben Rhodes told reporters.

"But we have no closer friend in the world and if we are asked our view as a friend, we'll offer it." Rhodes said Obama would be 'very straightforward and candid as a friend as to why the US believes that it is good for the UK to remain in the EU'.

Opinion polls suggest the June 23 vote will be a close one, but large numbers of voters remain undecided and Britain's allies are keen for Europe's second largest economy to stay in the bloc.

Aside from meeting British officials, Obama will have lunch with Queen Elizabeth II on April 22, one day after the long-serving monarch's 90th birthday.

He will also stage a joint news conference with Prime Minister David Cameron, who is campaigning for a vote to remain in Europe despite broad support for 'Brexit' within his Conservative Party.