The UK's withdrawal from the European Union (EU) would be a "disaster", a European commissioner has said.
EU Trade Commissioner Cecilia Malmstrom was asked about the implications of the UK voting to leave the EU in a national referendum.
Asked whether it would be a "disaster" if the UK voted to quit the EU, she replied: "Yes. For us and for you."
Ms Malmstrom is in London to brief ministers on the progress of EU-US trade talks.
The Conservative party has promised to hold an in-out referendum on the UK's continuing membership of the EU if it wins this year's general election.
The referendum would only be held after David Cameron, if he is still prime minister after May, has attempted to renegotiate the terms of the UK's membership of the EU.
Ms Malmstrom's admission was unusually frank for an EU commissioner, observers say. In the past, European officials, as well as other European heads of state, have simply said the decision is a matter for the British people.
Known as TTIP - the Transatlantic Trade and Investment Partnership - the EU-US trade deal would create the largest free-trade area in the world.
The British government maintains TTIP could add £10bn to the UK economy, £80bn to the US and £100bn to the EU every year.
It says shoppers would benefit by the removal of EU import tariffs on popular goods, such as jeans and cars. The government also argues that reducing regulation would help UK businesses export to the US, with small businesses in particular predicted to benefit.
At Monday's meeting, Business Secretary Vince Cable and UK Trade Minister Lord Livingston were due to ask Ms Malmstrom to give senior UK parliamentarians access to the TTIP treaty text, in order to allow them to monitor progress and ask questions on the public's behalf.
Several concerns have been raised about the treaty, including its potential impact on the National Health Service (NHS), which opponents in the UK say could lead to health services being privatised.
Mr Cable said he disliked "the level of secrecy that has surrounded the transatlantic trade deal so far" and could understand why some people were worried about the deal.
He added: "I will be working to ensure all British interests are protected and that the deal can be properly scrutinised."
"We must also clearly demonstrate that the NHS and our public services are protected as a priority. The EU has recently given us very strong assurances that TTIP would not in any way endanger them. I want to see that reflected in the treaty drafting."
But trade union Unite said that legal advice it had received from an expert in EU law and healthcare services suggested there were "clear dangers" arising from TTIP that would affect the NHS.
Unite general secretary Len McCluskey said there was "a clear risk of irreversible privatisation", because the trade deal would give US corporations or investors the right to sue the government if it ever tried to take back services into public ownership.
A government spokesperson said that TTIP posed no threat to the NHS whatsoever.
"Recent scaremongering suggesting TTIP could somehow force future governments to privatise public services is simply untrue and deeply irresponsible," the spokesperson said.