Former prime minister Tony Blair has said leaving the EU would create "an enormous economic problem" following any Brexit vote on 23 June.
And an online survey of economists for The Observer showed 88% of those who responded believed that EU withdrawal would be damaging for the UK economy.
There was a 17% response rate to the survey, from 639 economists.
The Vote Leave campaign said economists had been wrong in the past about whether the UK should join the euro.
"There was a cosy consensus among economists supporting Britain scrapping the pound 15 years ago... they were wrong then and they are wrong now," Matthew Elliott, chief executive of Vote Leave, said in a statement.
Speaking on the BBC One Marr Show, former Labour leader Mr Blair, a Remain supporter, said "if we vote to leave we will suffer an immediate shock to our economy" and "years of uncertainty".
He said this was not a hypothetical risk but "something you will see directly in people's jobs and living standards" and in the confidence of business to trade.
"What is now clear... is that if we did vote to leave, the economic aftershock would be severe," he added.
Meanwhile, according to those economists who replied to the Ipsos-Mori poll for The Observer, leaving the EU single market would harm the UK's growth prospects over the next five years.
Those who took part and responded are members of the Royal Economic Society and the Society of Business Economists.
Major reasons given by the economists as to why the UK would suffer were "loss of access to the single market" and "increased uncertainty leading to reduced investment".
Britain Stronger in Europe campaign director Will Straw said: "This is the final nail in the coffin of the Leave campaign's economic credibility.
"It is becoming clear that leaving is a risk we simply cannot afford to take."
Labour's shadow home secretary Andy Burnham also told Sky News that the UK would be damaged by leaving the EU.
However, Leave campaigners argue the UK would be freed from the cost of regulation and red tape imposed by Brussels if it left the EU.
They also say it would able to negotiate its own trade deals.
In The Sun newspaper on Saturday pro-Brexit government minister Priti Patel criticised Treasury forecasting of what would happen after a withdrawal from the EU, calling it "doom-laden propaganda".
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