US vice president Joe Biden has held talks at Downing Street amid renewed warnings from the Obama administration that it does not want Britain to leave the European Union.
He sat down with prime minister David Cameron on the final leg of a short tour of Europe that will also include taking part in a National Security Council meeting.
Earlier he said America's relations with the UK were "the most open" it enjoyed with any nation as he chatted with deputy prime minister, Nick Clegg.
The EU issue has gained wider international interest after Mr Cameron's promise last month to hold an in/out referendum if he wins the next general election.
In a pre-visit interview with The Times, Mr Biden said the White House view was that the UK was "stronger as a result of its membership.
"And we believe the EU is stronger with the UK's involvement."
President Barack Obama has already told the prime minister in a phone call that the US "values a strong UK in a strong European Union".
Mr Clegg made no mention of the issue as he welcomed Mr Biden to Number 10 - though he did hail the prospect on progress towards a US/EU trade deal.
He said a tie-up - which Mr Cameron is championing through the UK's presidency of the G8 this year - could provide a "significant boost to prosperity and jobs on both sides of the Atlantic.
"It's something which has been talked about a lot for many, many years but has been an elusive project to get going," he said.
The two countries should "work together to overcome the obstacles on both sides", he added.
Mr Biden said he welcomed the chance to discuss foreign policy issues such as Afghanistan, North Africa, Iran and the Middle East where "we both have a lot at stake".
He and Mr Clegg had "established a great relationship" over the course of regular contacts, he said, "and the most open relationship we have with any nation in the world is with Great Britain".
While in Germany at the weekend, Mr Biden said the US would be prepared to hold direct talks with Iran when its leadership demonstrated it was "serious".
He has also praised the French military action in Mali which Britain is supporting with the deployment of 200 military advisers to help train a West African intervention force.
Asked whether Mr Cameron would use the opportunity of Mr Biden's visit to outline his position on Europe to the vice president, the PM's official spokesman said: "They will certainly be discussing EU/US trade.
"As I understand it, the US position that the administration has expressed is a strong, outward-looking EU with a strong Britain inside that. That is very much our position."
On international issues, the PM and vice president can be expected to "discuss our shared agenda around international co-operation and how the UK and US continue to work closely together on a whole series of issues", said the spokesman.