One of the aims of the change was to simplify the system, a measure which should have improved take-up.
Figures published yesterday for 1989 - the first year of the Fowler reforms - show that pounds 1.65m in benefit went unclaimed, against pounds 1.425m in 1987, the last year of the old system. An estimated 1.6 million potential recipients failed to claim at a time when unemployment was falling, against 1.54 million in 1987.
But statistical problems and the changes introduced mean that valid comparisons of take-up between the old system and the new cannot be made, according to the Department of Social Security - leaving Peter Lilley, Secretary of State for Social Security, unable to show whether take-up has improved or not.
Donald Dewar, Labour's social security spokesman, said it was 'alarming' if such comparisons could not be made - and equally worrying that 1.3 million people did not claim the income support to which they were entitled.
The raw estimates appear to show that overall take-up of housing benefit - whose availability was restricted - improved, and that more of the cash in family credit was claimed by more of those entitled than under the old family income supplement. However, only 87 per cent of income support was claimed by 75 per cent of potential recipients, against 90 per cent of the cash being claimed by 81 per cent of the recipients under the old supplementary benefit.
However, Government statisticians state that the discontinuity in the series means 'any comparison between post-reform take-up rates for a particular benefit and its pre-reform counterpart would not be valid'.
Mr Lilley said the figures were 'encouragingly high', showing pounds 9 out of every pounds 10 available was claimed by almost four out of five of those eligible.
Within that, however, the figures show that the estimated 15 per cent of couples with children who failed to claim income suport lost pounds 32 a week.
Income-related benefits - estimates of 1989 take-up; HMSO; pounds 4.Reuse content