Britain's economy could shrink by 35% in the second quarter and see unemployment jump by two million, according to a scenario published by the Office for Budget Responsibility (OBR).
The UK fiscal watchdog published an estimate of the potential impact based on a three-month lockdown followed by a three-month period when restrictions are only partially lifted.
It sees GDP bouncing back quickly after its sharp contraction in the April-June period but estimates that the jobs market will take longer to recover.
The big increase in joblessness would take the unemployment rate up to 10%, a level not seen since the early 1990s. The rate currently stands at 3.9%.
The OBR scenario sees government borrowing for 2020/21 climbing by £218bn to £273bn, or 14% of GDP - the largest single year deficit since the Second World War.
That is based on an assessment of the devastating economic impact of the lockdown on tax receipts as well as the additional spending by the government to try to address the crisis.
The OBR stressed that its figures represented a "scenario rather than a forecast" based on the assumption that people's movements would be "heavily restricted for three months and would get back to normal over the subsequent three months".
They illustrate the likely scale of the challenge facing chancellor Rishi Sunak who has pledged to do "whatever it takes" to see UK households and businesses through the crisis.
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