The poorest households in the UK are failing to claim more than £5bn a year according to government figures.
Statistics from the Department for Work and Pensions prove show the take-up levels of income support, housing benefit, council tax benefit and jobseeker's allowance all fell in 2000-01.
Opposition parties claimed yesterday that the publication of the figures, due originally in September 2001, was timed to be overshadowed by the war on Iraq.
Steve Webb, an outspoken Christian and the Liberal Democrat work and pensions spokesman, said: "It is particularly shocking that over half a million pensioners fail to claim what should be theirs by right."
Means testing would be extended to nearly half of the population after new tax credits came into force this April and the pension credit is introduced in October, he added.
David Willetts, the shadow Secretary of State for Work and Pensions, said the figures were "devastating", and said that ministers must take the blame because "every time they change the benefits system or extend means testing, take-up falls."
However Andrew Smith, the Work and Pensions Secretary, pointed out that 170,000 more pensioner families were receiving minimum income guarantee, up one percentage point. Take-up figures were higher among the poorest pensioners.