US president Barack Obama will arrive in the UK for a visit which will see him drawn into the bitter row over the UK's membership of the European Union.
The White House has indicated that Mr Obama will set out his support for the UK staying in the EU, in a major boost to David Cameron and the Remain camp.
Mr Obama and the Prime Minister will hold a joint press conference on Friday where he is expected to give his opinion on the June 23 referendum despite calls from Leave campaigners for him to stay out of the debate.
Ahead of the visit, deputy national security adviser Ben Rhodes told reporters that the president would set out his support for a Remain vote if he was asked.
"He will make very clear that this is a decision for the people of the United Kingdom to make, it's not a decision for us to make.
"But we have no closer friend in the world, and if we are asked our view as a friend, we will offer it."
That would mean "being very straightforward and candid as a friend as to why the United States believes that it is good for the United Kingdom to remain in the European Union".
During the visit, Mr Obama will also have lunch with the Queen and hold a "town hall" style meeting with young Londoners.
Prominent Leave campaigners warned the president not to back continued UK membership of the EU ahead of his arrival.
London mayor Boris Johnson said it would be "hypocritical for America to urge us to sacrifice control - of our laws, our sovereignty, our money and our democracy - when they would not dream of ever doing the same", while Commons leader Chris Grayling suggested the US president "perhaps doesn't understand" the shift in power that has taken place between the UK and Brussels.
But former deputy prime minister Nick Clegg used a speech at a prestigious US university to urge Mr Obama to make the case for a Remain vote "loudly and clearly" during his visit to the UK.
Speaking at Princeton University on Wednesday, Mr Clegg warned that the "special relationship" would be put at risk if the UK left the EU.
Mr Clegg said the only prominent American who did not support the UK remaining in the EU was Republican presidential hopeful Donald Trump.
The former Liberal Democrat leader said during his time in government he was told "over and over again by US decision makers" that British influence in the EU "strengthened the West" and the co-operation between the EU and Nato.
"President Obama has said that having the UK in the European Union gives the States 'much greater confidence about the strength of the transatlantic union'. It is a case I hope and expect he will make loudly and clearly when he visits London in the coming days," Mr Clegg said.
"In fact, the only prominent American who has dissented from that view is Donald Trump.
"So, the choice for any British voter who cares about American opinion couldn't be clearer: Trump wants Britain out of the EU; president Obama wants us to stay in. It is one of the more remarkable twists in this ongoing saga that Boris Johnson, a contender for the future leadership of Britain's most Atlanticist political party, should side with Trump rather than Obama. I know whose side I - and I suspect millions of other Brits - would rather be on."
Mr Clegg added: "Britain and the United States will always be cousins, but we may not always be partners. If we leave Europe behind, America may leave us behind too."