Haringey Council has written off almost £19million of uncollected council tax debt since 2009, recently revealed figures show.
The authority wiped the outstanding cash from its books after accepting the debts, some of which dated back to 1993, were irretrievable.
Figures uncovered by the opposition Liberal Democrat group show that debts of £1.1miliion and £2million were written off in financial years 2009/10 and 2010/11 respectively.
But major data cleansing in 2011 revealed that much larger debts dating back more than 20 years remained unaccounted for.
The authority subsequently decided to write off £6.3million in 2011/12 and a further £9.2million in 2012/13.
A council spokesman said the decision was taken at a corporate level that such historic debts were unlikely to be retrieved.
Liberal Democrat group leader Councillor Richard Wilson accused Labour of not treating council tax as a priority, whilst the Labour group said the issue was non-political.