The National Audit Office (NAO) said £6bn was lost in 2012-13 due to debt being written off or not being pursued on value-for-money grounds.
More than £32bn had been lost since 2008-9, according to Government accounts, and departments allowed debt to go uncollected for too long, the NAO said.
The Government does not produce an official figure for how much it is owed by debtors, but the NAO estimated it was at least £22bn last March.
The level of debt owed to the Exchequer meant the Government had to borrow more, the NAO said.
HM Revenue and Customs was owed £15.1bn, with the bulk of the remainder owed to the Department For Work And Pensions and Ministry Of Justice.
The NAO report said nearly 70% of the losses were due to insolvencies.