The government borrowed £62.1bn in April, the highest monthly figure on record, according to the Office for National Statistics (ONS).
It underlines the cost of coping with the coronavirus pandemic through jobs schemes and loans, and is higher than many economists forecast.
The cost of furlough schemes had the largest impact, hitting £14bn in April.
The government’s independent forecaster, the OBR, has said the full-year deficit could hit £298bn.
April’s figure is significantly higher than analysts had predicted, with a consensus of economists predicting £30.7bn for the month.
Jonathan Athow, deputy national statistician at the ONS, described April’s figure as „pretty much unprecedented“.
He told the BBC: „Borrowing now is about six times what it was [in April] last year, so we are talking about some really significant changes in the government finances.“