The Institute for Fiscal Studies (IFS) has published a review of council tax support, five years after responsibility for it was moved from central government to local authorities in England.
The IFS estimates that around a quarter of the additional tax liabilities arising from the change remained uncollected in 2017-18.
In April 2013 the council tax benefit, which provided help for low-income households, was abolished.
Local authorities were made responsible for creating their own support schemes for those of working age, while also being required to provide a largely protected level of support for pensioners, which was determined by central government.
At the same time as moving responsibility for council tax support to local government, the central government also reduced the funding available for it.
Most councils consequently ensured that the new schemes were less generous than the old schemes, leading to some low-income households having to pay the tax for the first time, and others already paying it seeing their tax payments increase.
The 3.6m working-age households who would have been entitled to some support under the old system are now entitled to 24% less on average than if the earlier system had been maintained.
The report also says that there are now 1.4m households who have to pay some council tax who would not have had to pay it under the old regime.
Council tax support was paid to 4.9m households in 2017-18, and cost local authorities £4.1bn, equivalent to a reduction of about 11% of gross council tax bills.
There is a great deal of variation in the way support schemes are run.
The most widespread change since the 2013 reform has been the introduction of minimum council tax payments, but the details differ enormously.
The most common level of minimum payment is 20% and has been adopted by around one quarter, but a fifth of councils have no minimum payment and another fifth have minimum payments of over 20%.
