The UK Government was berated last night for "penalising" some of the nation's poorest pensioners after it emerged that thousands of them could lose hundreds of pounds each in a benefits' shake-up.

However, Whitehall insists the change is not about saving money for the Treasury but about simplifying the claiming of benefits for the most vulnerable in society and giving them extra support.

From October 6, those applying for pension credit, housing benefit or council tax benefit will only be able to have their claims backdated for three months instead of the 12 months that is currently the case.

It is estimated that around 100,000 pensioners could be affected by the change, with some potentially losing out as much as £2000.

The government insists the £100m a year saved through limiting the backdated payments will help pay for its planned reform of enabling pensioners to claim their entitlements by means of a single phone call and without having to fill in a complicated form. Ministers also point out that the savings will help them to increase the pension credit and winter fuel payment.

Mike O'Brien, the Pensions Minister, explained: "Many pensioners are still not claiming benefits they may be entitled to. That's why we are making every effort to urge pensioners who may be eligible to claim and making the process as easy as possible.

"The change will not affect existing customers and new customers can still claim 12 months' backdating if they apply before October 6. The changes are part of a wider package of measures to make claiming benefits simpler and more automatic and increase take-up," he added.

Pension credit was introduced in 2003 and is meant to ensure that pensioners should not be living below the breadline, guaranteeing an income of £124 a week for single people and £189 for couples.

Almost half of all pensioners are entitled to claim some form of benefit yet last year more than £2.5bn of available credit went unclaimed.

Last night, the Department for Work and Pensions stressed that 70% of people who claimed backdated benefits did so for three months or less. Yet official figures show that around 110,000 pensioners claim arrears for more than three months while 70,000 make backdated claims for the whole year. The average backdated claim in 2006/07 was £700, but some could claim as much as £2000.

John Barrett, the Liberal Democrat spokesman on pensions, said: "With these changes, the government is giving with one hand and taking away with the other. I cannot see any justification for the change in the backdating rules other than to save money for the Exchequer."

The Edinburgh West MP argued that the reality was that some of the most vulnerable pensioners would miss out on hundreds or perhaps thousands of pounds as a result of the planned change.

"The government should be focusing on increasing the benefits take-up rate rather than penalising pensioners, who were either unaware of the help that is available or unsure of how to apply," he added.

However, Mervyn Kohler from Help the Aged said that the new policy was a good package and was a "trade-off that is worth having".

He pointed out that if people lost a £1000 windfall through not having the pension credit backdated for a year, they were likely to gain around the same amount through automatically receiving council tax benefit.

He added, though, that the group was urging pensioners to submit claims for pension credit before October 6 so that they could benefit from the full 12 months' backdating.