Britain's looming exit from the European Union is another huge setback for negotiations on a massive US-EU free trade deal that were already stalled by deeply entrenched differences and growing anti-trade sentiment on both continents.

The historic divorce launched by Thursday's vote will almost certainly further delay substantial progress in the Transatlantic Trade and Investment Partnership (TTIP) talks as the remaining 27 EU states sort out their own new relationship with Britain, trade experts said on Friday.

With French and German officials increasingly voicing scepticism about TTIP's chances for success, the United Kingdom's departure from the deal could sink hopes of a deal before President Barack Obama leaves office in January.

"This is yet another reason why TTIP will likely be postponed," said Heather Conley, European program director at the Center for Strategic and International Studies, a think tank in Washington.

"But to be honest, TTIP isn't going anywhere, I believe, before 2018 at the earliest," she said.

Both the US Trade Representative's Office and the trade office of the European Commission, the EU's executive body, declined to comment on the implications of the "Brexit" vote.

TTIP negotiators are still expected to meet in Brussels in mid-July as scheduled, but those talks were aimed at focusing on less controversial issues while leaving the thorniest disagreements for US and EU political leaders to resolve.

And it is unclear when Britain will launch formal separation proceedings, which will take at least two years.

But analysts said both sides have been reluctant to put their best offers on the table with a new US president due to take office in January and French and German leadership elections nearing in 2017.