Britain will lose up to £66 billion a year if it pursues the so-called "hard Brexit" option of leaving the single market and EU customs union, the Treasury has warned.
Government figures suggest the UK's gross domestic product (GDP) could fall by as much as 9.5 per cent if it leaves the EU and reverts to World Trade Organisation rules.
The impact of such a slump would be devastating on the public sector, according to Treasury documents leaked to The Times.
The draft Cabinet committee paper is based on a controversial study published by George Osborne in April during the referendum campaign. But despite the vilification it received then, the Treasury says it still stands by the figures now.
It comes as David Davis, the Brexit minister, continued to signal his preference for a hard Brexit even as the value of the pound dropped below the euro in some currency exchanges.
Though EU leaders have suggested Britain will not be allowed to leave the bloc with a better deal than it had as a member, Mr Davis said: "It is not necessary to be a member of the single market, to trade incredibly successfully in the single market."
The leaked government document says: "The Treasury estimates that UK GDP would be between 5.4 per cent and 9.5 per cent of GDP lower after 15 years if we left the EU with no successor arrangement, with a central estimate of 7.5 per cent."
It adds: "The net impact on public sector receipts - assuming no contributions to the EU and current receipts from the EU are replicated in full - would be a loss of between £38 billion and £66 billion per year after 15 years, driven by the smaller size of the economy."
Brexit backers who have seen the documents told the newspaper the figures were unrealistic and claimed there was a push to "make leaving the single market look bad".
But prominent Remain campaigners pushing for a "soft" Brexit that would keep Britain in the single market said the documents showed the "horrific damage" of leaving the trading bloc.
Conservative former minister Anna Soubry, a supporter of the Open Britain campaign, said: "The horrific damage of a hard Brexit is clear. Less tax revenue means less to invest in schools and hospitals, lower trade and investment means businesses and jobs at risk.
"This danger is precisely why Parliament must be involved in the principles to guide the Brexit negotiations.
"Britain will leave the EU, but we must do so in a way the protects our prosperity and reduces risk. The Government should now make clear the 'WTO option' isn't on the table."
Liberal Democrat leader Tim Farron said the leaked documents showed quitting the single market would wreck the economy.
"This is yet more proof that hard Brexit would be an act of sheer economic vandalism," he said.
"The Liberal Democrats will stand up for Britain's membership of the single market.
"We cannot stand by while this reckless, divisive and uncaring Conservative Government wrecks the UK economy."
A Government spokesman said: "We want the best outcome for Britain. That means pursuing a bespoke arrangement which gives British companies the maximum freedom to trade with and operate in the single market, and enables us to decide for ourselves how we control immigration."
Additional reporting by agencies
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