More than half the 1.5 million people claiming incapacity benefit in Britain are overweight, according to new figures released by the government.

The cost to the taxpayer of the growing obesity epidemic is now put at more than £6bn a year - more than double previous estimates.

Official figures released by Lord Warner, the parliamentary under-secretary at the department of health, show that 900,000 people claiming the benefit have been diagnosed as obese.

Many of them suffer from diseases such as diabetes, which have been linked to being overweight.

According to the figures the 900,000 each receive, on average, benefit of £78.85 a week, bringing the annual bill to about £3.6bn.

This amount is in addition to estimates on the cost of obesity released last year by the National Audit Office, Parliament's financial watchdog. The office suggested the problem was costing the taxpayer £2.6bn a year, an amount expected to increase to £3.6bn by 2010.

This included a £500m bill for the NHS and £2.1bn in lost earnings caused by premature mortality and sickness absence.

An audit office spokesman told the Guardian: "These figures did not include claims for benefit - apart from when people were off work sick.

"The new figures are also more up to date than the NAO figures since they refer to the situation in 2003 - two years after the figures supplied to the NAO for their report."

Seventy per cent of men and 63% of women were estimated to be overweight in 2002.

Last week the health secretary, John Reid, called for better labelling of fatty foods as part of a drive to tackle obesity.

Alarm is mounting in the Treasury and the Cabinet Office that a failure to slim the nation could lead to a public spending crisis in Whitehall.

Unlike the United States where the costs of medical treatment caused by obesity are borne by private health insurance and employers, in Britain the bills will have to be met from public funds.