No Deal Brexit would trigger a year-long UK recession in just a few months' time, the government's official spending watchdog has predicted.
In the first forecast of its kind, the Office for Budget Responsibility said a recession would begin in the final three months of 2019 if the UK leaves without agreement.
The pound would drop in value instantly by 10% - while unemployment would rise by more than a quarter.
GDP would fall 2% by the end of 2020, around the same as the early 1990s recession and a third of what was seen in the 2008 financial crisis.
In all No Deal would add about £30billion a year to government borrowing from 2021, the OBR said - hiking the UK's net debt by 12% of GDP by March 2024.
It comes hours after Boris Johnson - who has vowed to leave the EU deal or no deal on October 31 - said he would ramp up planning for No Deal after taking office.
The imminent Prime Minister has refused to rule out suspending - "proroguing" - Parliament to force through No Deal.
Before he even takes office next week, MPs will stage a crunch vote today in Parliament in a bid to stop him.
Despite the drastic findings, OBR forecasters said their stress test was "by no means a worst-case scenario".
It includes compromises, including temporary recognition of standards and temporary tariffs, before assuming the UK then reverts to EU "most favoured nation" tariffs later on.
The OBR said in its prediction, GDP would fall due to lower business investment triggered by "heightened uncertainty".
And unemployment would rise from its current record low of 3.8%, peaking at just over 5% in 2021.
"This is a smaller rise and a lower peak than in either of the past two recessions," the OBR said.
"Employment then recovers and the unemployment rate is only slightly above our March forecast by the first quarter of 2024."
The OBR said the "nominal sterling effective exchange rate" would drop "immediately" by 10%, but end up around 5% lower in real terms by March 2024.
Meanwhile the OBR added: "The UK enters a year-long recession in the fourth quarter of 2019.
"Real GDP falls by 2.1%, around the same as in the early-1990s recession but only about a third of what was seen in the financial crisis.
"By mid-2021, real GDP is 4.0% lower than in our March forecast. The economy then picks up and is just 1.6% smaller than the baseline by the first quarter of 2024."
The OBR is an independent watchdog which advises the UK government on government spending and financial risks. The stress test was based partly on work by the International Monetary Fund.
This breaking news story is being updated.
