Chancellor George Osborne has defended claims an EU exit would cost households an average £4,300 a year - after Leave campaigners said it was "absurd".
A Treasury analysis says the UK economy would be 6% smaller if it left the EU than it would otherwise be by 2030.
This would leave a £36bn hole in the public finances said Mr Osborne, who called the report "serious and sober".
But exit campaigners said the estimates were "worthless" and "unbelievable" given the Treasury's past record.
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Former Conservative Chancellor Lord Lamont, of Vote Leave, said: "The chancellor has endorsed a forecast which looks 14 years ahead and predicts a fall in GDP of less than 0.5% a year - well within the margin of error.
"Few forecasts are right for 14 months, let alone 14 years. Such precision is spurious, and entirely unbelievable."
Meanwhile, Conservative MP John Redwood, also campaigning for an Out vote, pointed out the Treasury had "failed to forecast the huge damage membership of the European Exchange Rate Mechanism inflicted on us", as well as the impact of the 2011 Eurozone crisis.
"I think their 2030 forecast is completely worthless," he said.
But Mr Osborne told BBC Radio 4's Today: "The conclusions could not be clearer. Britain would be permanently poorer if we left the EU to the tune of £4,300 for every household in the country. That's a fact everyone should think about "
Analysis, BBC political editor Laura Kuenssberg
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