Legal & General has seen a record rise in workplace annuities as it offsets the fall in individual annuity sales driven by the Budget pensions shake-up

Retirees risk being misled by estimates proposed by ministers which would tell them when they are expected to die unless this information is regularly reviewed, a leading pensions expert has warned.

Pensions Minister Steve Webb has suggested that giving people who are nearing retirement an idea of how long they might live for would help them make informed financial decisions.

But Ros Altmann, an independent pensions expert and a former Downing Street adviser, said the information could be "not terribly helpful" and "a bit misleading" unless the figures are regularly revised over the course of someone's retirement to take account of any life changes and backed up by detailed guidance.

She said: "Unless you've got some process where you update the figure it becomes a bit meaningless.

"By the time five or 10 years have gone by, things can change by a big amount for lots of people."

Experts could look at factors such as smoking, eating habits and someone's social and economic background when determining approximate life expectancy.

The proposed guidance, which could be rolled out in April next year, would form part of a major shake-up of the pensions system outlined in the Budget, which aims to make it easier for people to cash in their pension pots rather than using them to buy a lifetime income called an annuity.

Controversy over annuities has been growing in recent years amid plunging rates and fears that people are not shopping around enough to get the best deal.

The new flexibility that the retirement reforms will bring to people has sparked concerns that some people may under-estimate how long they will live for and burn up their savings too quickly.

Last month, Mr Webb was criticised for being "out of touch" after saying he was "relaxed" about people wanting to use their money to buy a Lamborghini.

Dr Altmann said that the principle that people should not under-estimate how long they will live for "is of interest" and perhaps someone aged 60 does need to know that the likelihood is they will live for around another 20 years.

But she also pointed out that five years later that person could experience a life-changing event such as a heart attack that could shorten their life expectancy.

She said: "Unless you have a regular estimate, a one-off number is not going to be terribly helpful or meaningful. It could be a bit misleading.

"You will have some people being very cautious who might suddenly die or get very ill, or you might have others who say 'I'm going to spend it now' who hang on to their late 90s.

"Unless there is proper information, help and guidance it is not going to really achieve what (Mr Webb) would like it to achieve."

Dr Altmann predicted that the proposed lifespan estimates would be likely to be based on similar "rough and ready" calculations to those used to work out what level of annuity people should get.

She also pointed out that people need to bear in mind that their income needs will change over the course of their retirement, describing them as "J-shaped" if they were plotted out on a graph.

Explaining the J-shape, Dr Altmann said that at the start of someone's retirement their income needs tend to remain relatively high, and they then drift downwards as the person becomes less active and spends more time at home. But towards the end of their life, that person may have care needs which could be "very expensive", Dr Altmann said.

She said: "(Their income) may need to shoot up and that needs to be factored in."

Life expectancy is rising steadily in the UK, with girls born now projected to live until 82.7 years on average. Average male life expectancy is now 78.9 years.

The coast of Dorset fared best for women, with girls born in the Purbeck area expected to live to 86.6 years and men in neighbouring east Dorset living to 82.9 years.

Glasgow had the lowest figure for boys and girls at 72.6 years and 78.5 years respectively.