Saying that you're too busy to exercise is no longer a valid excuse - researchers have found that only 15 minutes a day can extend your life by three years.

A new study published on Tuesday on the Lancet medical journal website also found that one in nine cancer-related deaths "could have been averted" if the patient had done 15 minutes of "moderate-intensity daily exercise".

Chief researcher Chi-Pang Wen, of the National Health Research Institute, in Taiwan, and the China Medical University Hospital, said the study could help people to live a healthier life.

"Patients might be more easily motivated to exercise if their doctor recommends an easily manageable [duration]," he said.

The 12-year study between 1996 and 2008, with 416175 Taiwanese subjects - categorised people depending on how much exercise they did, and measured their life expectancy and chance of dying of cancer.

People who have a high risk of heart disease and heart attacks could "benefit substantially from incorporating this low level of exercise" into their daily life, the researchers found.

Though the World Health Organisation recommends 30 minutes of exercise a day for "substantial health benefits", the Lancet study shows that people who take baby-steps to get fit can also live longer.

"Barriers exist in meeting this 30 minutes a day, five days a week recommendation," the study said.

Exercising for 90 minutes a week added three more years onto a woman's life and two-and-a-half onto a man's, the study found.

People who exercised for 30 minutes a day, according to the guidelines, added between three and four years to their lives.

Starting with just a little exercise could encourage people to do more, the researchers found.

"Once an individual does 15 minutes of daily exercise regularly, he is more likely to increase the time spent exercising," the study said.

Sherman Baatjes, fitness lecturer on Stellenbosch University's department of sports science, said: "One can never say that we are too busy to exercise."

"The beginning phase of any physical activity revolves around the mere fact that you are starting something new or different."

He suggested starting by taking a walk during the lunch break to "get some fresh air into the lungs".

"Such an activity would promote lower-limb muscle endurance, which is one of the cornerstones of physical fitness exercise," he said.

When walking became easy, Baatjes said, it could be "replaced by jogging, cycling, or swimming".