The UK's decision to leave the EU has caused "deep concern" in the United States about the future of the transatlantic relationship, the EU ambassador in Washington has said.

David O’Sullivan said the UK referendum result left the US, which wished to maintain the best of relations with the UK and the rest of the EU, in the “not very comfortable position of trying to figure out how we’re going to deal with this”.

He warned that the uncertainty surrounding Brexit was "our biggest enemy" and would have "a hugely chilling effect" on investment decisions, at least in the short term.

“This is going to take time to work through. We have never done this before. No one has a perfect blueprint about how you might go about it,” he said. “At the same time I think what people do want and hope for in the coming months is at least some clarity about the process.”

O'Sullivan, formerly the most senior civil servant in the European Commission, said many in the US were "deeply concerned" about what Brexit meant for transatlantic co-operation in areas such as trade, security and intelligence-sharing.

“How this will affect the transatlantic relationship is something of deep concern to American colleagues,” he told a seminar at the Institute of International and European Affairs in Dublin yesterday.

Arguing that the US-EU relationship was "in very good shape", O'Sullivan pointed to the Iran nuclear deal, the Paris agreement on climate change and co-operation on issues such as Ukraine and terrorism as positive signs.

“Of course, we have a general election in the United States, and the single biggest determinant of the future direction of those relationships will be the outcome of that election.”

TTIP deal

Notwithstanding some public opposition, O’Sullivan said the EU-US trade deal known as TTIP (Transatlantic Trade and Investment Partnership) could still be agreed by the end of the year. “It’s challenging but not impossible,” he said of that timeline. But even if it is not achieved this year I do believe that this deal will be done, and I believe that it needs to be done.

“The transatlantic economic corridor is the largest and most important corridor in the world by far. It would seem extremely paradoxical to me that with all the deals we’re doing with other people, we could not do a trade deal with each other.”

Values

On the values that bound Europe and the US, O'Sullivan remarked that they were "probably the two major regions of the world who still hold to the fundamental values that underpin the world liberal order that was set up in the second half of the 20th century".

These values were under threat from China and Russia, which had "a different vision" of how the world should develop, but they were also under threat from within.

"I'm not going to comment on the United States election, but you have seen the people who are voting for Bernie Sanders, who are voting for Donald Trump. These are not necessarily people who are supportive of the global liberal order which has brought so much prosperity and security to the West and also more globally.

"You also see it in Europe. The Brexit vote was, of course, a vote about the European Union, but it contained elements of this unhappiness with globalisation, disillusionment with trade, with freedom of movement of persons.

“This is the big debate of the coming years, and I’m absolutely convinced that the United States and the European Union have a very important role to play in defending those values.”