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, AFP reported.
"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.
"There would be less trade, less investment and less business... Leave the EU, and the facts are: Britain would be permanently poorer," he said.
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 percent economic 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 agreement with Canada will remove most duties between the EU and Canada by 2023 and allow EU companies to bid for public contracts in Canada.
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.