It would be a "nightmare" if Britain decided to quit the European Union, a former director general of the World Trade Organisation has warned.
Pascal Lamy, who ran the WTO from 2005 until 2013, told The Huffington Post that Europe was operating in "total obscurity" about what powers David Cameron would need back in order to go into a referendum recommending that Britain remain a member.
"On 'Brexit', which is a problem for us all, I must say, seeing this country drifting in the direction would be a nightmare for many of us. There's not much we can do, like most nightmares, by the way," he said.
Lamy is a veteran of the EU's institutions and has previously served as EU trade commissioner as well as chef de cabinet to then European Commission president Jacques Delors.
"The whole question is what will Brexit mean? Frankly speaking nobody knows. And until we've seen some sort of UK government position on this, and we haven't see anything of this kind so far, it is very difficult.
"We know that something should be renegotiated with Brussels. But what? We don't have a clue. We have leaks, internal consultations, which by the way I'm properly informed [is a] very meagre list that would not satisfy at all the sort of Ukip approach towards negotiation."
The Frenchman added it was a "very difficult" question as to how the UK could "exit part of the EU system without exiting others".
His comments come amid reports up to half a dozen Tory cabinet ministers believe Britain should leave the EU if the prime minister fails to secure a major renegotiation before the planned 2017 referendum.
Lamy had been speaking at the London School of Economics about the free-trade agreement, Transatlantic Trade and Investment Partnership (TTIP), currently under negotiation between the EU and United States.
Ignacio Garcia Bercero, the EU's chief negotiator for TTIP who was speaking alongside Lamy, refused to comment on Britain's possible exit form the EU when asked.
However he told HuffPost that the areas of negotiation between Brussels and Washington that were proving most "challenging" were the ones that mattered most to London, including financial services.
"I find it very difficult how anyone could argue that UK on its own could achieve a better result on those areas than by being part of the EU," he said.
TTIP is highly controversial. It's advocates argue it will reduce the price of cars, clothes and other imports. However opponents have have that TTIP would lead to the privatisation of parts of the NHS. There are also concerns that the deal allows companies to sue governments over changes to policy.
Last July the US trade representative Michael Froman said the deal could be done quickly "on one tank of gas". However Lamy said it was "pie in the sky for anyone knowing anything about trade negotiations" to have thought agreement could be reached by the end of 2014.
"It will take at least 15 years," he predicted. He said negotiators need only look to how long it has taken for the EU to agree its own internal market, which began in 1985, and is not yet complete.
Cameron backs the deal and has dismissed fears that it will lead to health care being privatised. "There's no threat, I believe, from TTIP to the National Health Service and we should just knock that on the head as an empty threat," he recently said.
On Tuesday, Pope Francis attacked the European Union's "opulent" and "aloof" leadership and institutions in an extraordinary speech to the Strasbourg parliament.