The maximum length of human life is rising steadily and there may no limit to how long people can live, say researchers.

Scientists have previously suggested that the end limit of a human life is 120 years and that nobody could live longer.

But researchers in the US say the fact that the oldest humans are living longer than their predecessors suggests there may not be an upper limit.

They believe that improvements in health are not only helping people to survive into old age but are also helping elderly people to live longer.

Researchers from the University of California, in the US, analysed births and deaths in Sweden - a country with good records and seen as typical of industrial countries - over the past 240 years.

They found a constant trend upwards in the age of the longest living people.

For instance, the longest lived person born in 1756 died in 1857 at the age of 101. The longest lived person born in 1884 died in 1993 at the age of 109.

In the 1860s, the oldest age at death was 101. This increased slowly throughout the next century rising to 105 by the 1960s.

But it rose sharply over the next 40 years with the oldest lived person in the 1990s aged 108.

Professor John Wilmouth, from the University of California, said better sanitation, improved public health and safer water supplies have helped to extend human life.

"The elderly today are benefiting from the fact they were not as sick when they were children as in past generations and these changes took place 80 to 100 years ago."

He added: "After 1970, the trend began to slope upward rapidly. That corresponds closely with breakthroughs in certain medical practices such as understanding and treating heart disease and stroke."

Professor Wilmouth dismissed previous theories that the human life span cannot last beyond 120 years.

"There is no scientific basis on which to estimate a fixed upper limit. Whether 115 or 120 years, it is a legend created by scientists who are quoting each other."

He added: "We have shown that the maximum life span is changing.

"It is not a biological constant. Whether or not this can go on indefinitely is difficult to say.

"There is no hint yet that the upward trend is slowing down.

"Human progress is real, somehow. We are changing the limits of human life span over time."

A spokeswoman for Age Concern said attitudes and government policy would have to change to cater for a population that was living longer.

"We are already having to change our ideas about retirement as people are living longer.

"If we want retirement to be fulfilling there has to be a serious rethink of social policy particularly in terms of healthcare and education to help an older population," she said.

The study is published in Science journal.