Quitting the European Union would plunge Britain into a year-long recession, according to George Osborne.
The Chancellor warned that the country would suffer an "immediate and profound" economic shock if it voted in favour of Brexit on June 23.
His comments are part of Treasury analysis ahead of the referendum in just one month's time.
The analysis claims that economic growth would be at least 3.6% lower in the event of Britain leaving the EU, and in the worst case scenario could plummet as much as 6%.
Osborne's warning comes shortly after alerts by Bank of England governor Mark Carney and the International Monetary Fund that quitting would leave the UK in the red.
Mr Osborne, who is visiting a business on the south coast with Prime Minister David Cameron to set out details of the Treasury study, will warn voters not to inflict a "do-it-yourself" recession on the country.
The Chancellor is expected to say as follows:
Using "cautious" assumptions, with the UK entering into a new trade deal with the EU, the Treasury report found GDP would be around 3.6% lower after two years compared to the forecast for continued growth after a vote to remain.
A sharp rise in inflation would also be expected and house price growth faces being hit by 10%.
Under a "severe shock scenario", where Britain would leave the Single Market and default to World Trade Organisation rules, GDP would be 6% lower after two years and there would be a further increase in inflation, with a hit to house price growth of 18%.
The assessment found that even under the more conservative modelling, recession would be expected "with a significant risk that the outcome could be far worse" while remaining would see "current uncertainty fall back rapidly with little lasting impact on the economy".
Vote Leave said the analysis was "not an honest assessment".