Scientists and medical ethicists are warning of the dangers posed by a new blood test for determining how fast someone is ageing.

The £435 ($900) test measures the length of a person's telomeres, the structures on the tips of the chromosomes which get progressively shorter with age. Short telomeres are linked with age-related diseases and premature death.

The Spanish company behind the test, Life Length, is in discussions with a company that operates in Britain to market the test over the counter later this year.

The test's inventor, Maria Blasco of the Spanish National Cancer Research Centre in Madrid, said the test was accurate in detecting dangerously short telomeres, which are linked with age-related diseases and premature death.

"We know that people who are born with shorter telomeres than normal also have a shorter lifespan. We know that shorter telomeres can cause a shorter lifespan," Blasco said.

Experts, however, are worried that people may misunderstand the limitations of the test, which purports to measure a person's true "biological" age rather than the usual chronological age.

They are also concerned that the information may be used by insurance companies and organisations trying to sell fake anti-ageing remedies.

"I'm sceptical and concerned about this test mainly because of the lack of evidence that this information is useful and yet this test touches on really significant issues, such as predictions of life expectancy," said Colin Blakemore, an Oxford neuroscientist and former head of the Medical Research Council.

"My pressing concern is just how reliable these tests are in terms of anything significant. We need to know an awful lot more before we make predictive statements. People worry about how predictive it is."

Thomas von Zglinicki, a professor of cellular gerontology at Newcastle University, said it was not yet clear how accurate such telomere tests were when applied to individuals.

"To sell this to the public is premature and I will not buy it," Von Zglinicki said.

Medical ethicist Piers Benn, formerly of Imperial College London, said there were wider philosophical dangers of using a test that might estimate how long a person has left to live.

"If we knew when and how we will die, that would influence the way we lived; we shape our future in the light of the uncertainty in which we live," Benn said.

"We need to avoid the fatalism which says that I'm going to die on a certain date so why should I give up smoking or avoiding bad foods."

Life Length is expecting hundreds of requests from people wanting to have their telomeres tested and demand from thousands more once the company is able to bring down the cost of the test as public demand increases.

Although Life Length is not the only company selling telomere tests, it is the only one gearing up for over-the-counter sales to the public and the only company with an accurate enough test to be of practical use, says Professor Jerry Shay of the University of Texas Southwestern Medical Centre in Dallas.

"This test devised by Blasco is so accurate that it is likely to provide more useful information than some of the other tests out there right now," said Shay, who is a scientific consultant for Life Length.

"What's important in ageing is the shortest telomeres.

"What makes cells stop growing is the shortest telomeres, not the average telomere length, which is what other tests look at.

"Everyone talks about the chronological age, but there is also a biological age and telomere length is actually a pretty good representation of your biological age. Telomeres are important - there is no question of that."

Asked why the general public would be interested in taking a telomere test, Shay said: "I think people are just basically curious about their own mortality.

"If you ask people what they worry about, most people would say they are worried about dying."

He added: "People might say 'If I know I'm going to die in 10 years I'll spend all my money now', or 'If I'm going to live for 40 more years I'll be more conservative in my lifestyle'.

"The worrying thing is that if this information ever got to a point where it is believable, insurance companies would start requiring it in terms of insuring people.

"If you smoke or you're obese your insurance rates are higher, and if you have short telomeres your insurance rates might be higher too."

But Josephine Quintavale, of Comment on Reproductive Ethics, warned that such tests might be used to marginalise the elderly.

"Sadly, the elderly are already not the most popular members of society when it comes to healthcare allocation and I could definitely foresee a culture of not spending resources on those with short telomeres."

Telomeres: A history
2003: Scientists studying 20-year-old blood samples from 143 people show that telomere length is a good indicator of whether someone is likely to live for 15 years or more once they reach 60.

2004: Women living with the stress of having a sick child are found to have shorter telomeres. Other research suggests that meditation or other forms of stress reduction may lengthen telomeres.

2007: Study of men in Scotland shows those with the longest telomeres were half as likely to develop heart disease than those with shorter telomeres. Telomere length was as good as cholesterol levels at predicting the risk of developing cardiovascular disease.

2009: Short telomeres are linked with inherited bone marrow disease.

2010: GM mice with no telomerase, an enzyme that elongates telomeres in some cells, age prematurely compared with normal mice. The ageing effects were reversed after injections of telomerase.

2011: Study of civil servants in the UK shows that those with few educational qualifications have shorter telomeres than those with higher educational qualifications. People with poor backgrounds are known to age faster and suffer more age-related diseases.