The report said a Brexit would cause "permanent" economic damage as Britain would never be able to negotiate quota-free, no-tariff access to the single market if Britons vote to leave in a June referendum.
"The conclusion is clear: for Britain's economy and for families, leaving the EU would be the most extraordinary self-inflicted wound," Chancellor of the Exchequer George Osborne wrote in The Times.
Under all the Brexit scenarios examined, Britain would have a "less open and interconnected economy".
"It's a complete fantasy to suppose that there is some radically different other arrangement that Britain could negotiate, where we have access to the single market but don't accept any costs or obligations of EU membership," Osborne wrote.
The six per cent drop forecast was based on the assumption that in the event Britain left the bloc, it would negotiate a trade deal similar to the EU-Canada pact, according to extracts from the report.
The UK Treasury's 200-page report has been months in the making and is the latest stark warning from the government ahead of the June 23 referendum.
The polls show the two camps neck and neck, while around a fifth of voters remain undecided.
The run-up to the referendum is being closely watched across Europe and beyond because of its potentially far- reaching economic and political consequences.
The Treasury report is being published just days before US President Barack Obama is due in London on a visit in which he is expected to underline the importance of Britain staying in the EU.
International Monetary Fund chief Christine Lagarde also called on Britain and the EU to save a "long marriage".
The IMF last week downgraded its forecast for British economic growth by 0.3 percentage points to 1.9 per cent for 2016, although it held its 2017 forecast at 2.2 per cent.
Charismatic London mayor Boris Johnson, who is campaigning for Britain to leave the EU, wrote in the Daily Telegraph that the vote was on a "knife-edge".
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