The UK could face an extra two years of austerity measures if it votes to leave the EU, the Institute for Fiscal Studies has said.
The consensus of economists was that the UK economy would shrink after an EU exit, the think tank said.
It warned ministers could react to a post-Brexit GDP fall with either deeper cuts, or by extending them.
But UKIP leader Nigel Farage said the IFS was biased because it was part-funded by the EU.
He told BBC News: "They take direct funding from the European Union.
"So, once again, it's the same old game. It's taxpayers money being used to tell us what we should think and what we should do. And frankly the scale of this now is outrageous.
"The government and all their friends - taxpayer-funded friends - are frankly cheating in this referendum by selling this constant negative message."
IFS director Paul Johnson told BBC Radio 4's Today programme, his organisation got "about 10% of our income from something called the European Research Council, which is an independent arms-length body which funds world-class academic research".
But he added: "For the last 30 years, the IFS has really built its reputation on the independence and integrity of our work, and actually there is no sum of money from anywhere in the world which would influence what we said".
'Further austerity'
Prime Minister David Cameron said the IFS was the "the gold standard in independent, impartial economic forecasting and commentary in our country".
The IFS assessment comes as a dozen former senior military officers have warned that the EU's policies are undermining the UK's combat effectiveness.
Speaking out in favour of Britain leaving the EU, they said Nato, and not the EU, should remain the cornerstone of Europe's defence.
More than 300 historians, meanwhile, have written to The Guardian saying Britain has an "irreplaceable role to play in Europe" and warning against "condemning ourselves to irrelevance" by leaving.
The IFS said a vote to leave could result in a £20bn to £40bn hit to the public finances in 2019/20, if GDP was 2.1% to 3.5% lower over the period, as predicted by the National Institute of Economic and Social Research (NIESR).
Paul Johnson, IFS director and an author of the report, said: "Getting to budget balance from there, as the government desires, would require an additional year or two of austerity at current rates of spending cuts."
http://www.bbc.com/news/uk-politics-eu-referendum-36371700