Thousands of pensioners are set to lose hundreds of pounds in benefits when changes are introduced in October.

Charities have warned that the reforms, which will cut the amount of time pensioners have to claim backdated benefits, will mean that some of the most vulnerable pensioners will be left out of pocket.

From October 6, pensioners will only be able to claim backdated pension credits, housing benefit and council tax benefits for the previous three months, rather that the 12 months currently allowed.

The changes are part of a plan to allow pensioners to apply for all three benefits in one phone call without the need to fill in any forms.

It estimates that the changes will save government £100 million.

Last year over 110,000 pensioners made a backdated claim for a period longer than three months and 70,000 claims for the previous year. The average backdated claim was for £700.

Over 1.8 million pensioners who are eligible for pension credits did not claim in the last year, according to the Department for Work and Pensions (DWP), missing out on up to £2.8 billion in pension credits alone.

Gordon Lishman, Age Concern's Director General, said: “With up to £5 billion in benefits still failing to reach some of the poorest pensioners, the Government must work much harder to get benefits cash to those who are entitled to it, as soon as they become eligible.”

There are 2.1 million pensioners living in poverty, according to figures from the DWP, an increase of 300,000 in the last 12 months.

John Barrett, Liberal Democrat Work and Pensions spokesperson, 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,” he said.

The DWP has defended the move, arguing that thousands more pensioners will have access to benefits when the new system is introduced.

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

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