WARSAW (Reuters) - U.S. President Barack Obama urged German Chancellor Angel Merkel on Saturday to ensure an orderly transition to a close relationship between Britain and the European Union once the UK leaves the EU, a White House official said.

Meeting on the sidelines of a NATO summit in Warsaw, "the President and the Chancellor reaffirmed the need for an orderly process to determine the future relationship between the UK and EU," the official said. 

"They agreed that a highly integrated UK-EU relationship is in the best interests of the‎ transatlantic community."

EU leaders have said Britain can only retain full access to their lucrative single market, including for its large financial services sector, if it continues to allow free movement of EU workers and upholds EU rules. The two candidates to succeed Prime Minister David Cameron have both said they intend to restrict immigration.

(Reporting by Ayesha Rascoe; Writing by Paul Taylor)