Women in the study who took two to five aspirins a week were far less likely to die from breast cancer or for the cancer to spread
Women with breast cancer who take aspirin at least twice a week canmore than double their chance of surviving, researchers say.
The greatest protection comes from taking the drug two, three, four or five times a week, a study has found.
They cut the risk of dying by 71 per cent and the risk of the cancer spreading by 60 per cent.
Taking aspirin on six or seven days cut the death risk by 64 per cent, but the risk of spreading fell only 43 per cent.
The findings of the U.S. study provide the most compelling evidence yet of the power of the cheap painkiller.
Previous research has suggested that aspirin can protect againstbowel cancer, although results for other cancers, such as breast andprostate, were less clear-cut.
The latest dramatic results came from a 30-year project tracking the health of 238,000 nurses.
Lead researcher Dr Michelle Holmes, of Harvard Medical School,said: "This is the first study to find that aspirin can significantlyreduce the risk of cancer spread and death for women who have beentreated for early-stage breast cancer.
"If these findings are confirmed in other clinical trials,taking aspirin may become another simple, low-cost and relatively safetool to help women with breast cancer live longer, healthier lives."
Drugs in the same class as aspirin, including ibuprofen and naproxen, also lowered the risks, but paracetamol did not.
Expertswarned, however, that aspirin can have serious side effects, includingstomach irritation that can lead to ulcers and even fatal bleeding.
For some people the risk of harm is greater than potential benefits.
Women newly diagnosed with breast cancer are advised not to takeaspirin for the first 12 months as it can cause side effects while theyundergo chemotherapy or radiation.
Researchers are uncertain exactly how aspirin affects tumoursbut it could be by lowering inflammation. The study found that therewere no beneficial effects for people who took aspirin only once aweek.
Despite its benefits, many cancer sufferers could also find themselves struggling with severe side effects if they take aspirin regularly
Dr Holmes said: "Aspirin cannot be considered a substitute forconventional cancer treatments, and taking aspirin does have negativeeffects in some.
"More study is definitely needed toestablish the cause and effect of aspirin on breast cancer. But fornow, if a woman has breast cancer and is taking aspirin, she may takesome comfort in knowing she might be doing something to help preventher breast cancer from recurring."
Millions of people in the UK already take low-dose aspirinevery day on doctor"s advice to reduce the chance of a repeat heartattack or stroke.
Others take it of their own accord for "health insurance".
Most of the women in the new study, published in the Journal ofClinical Oncology, were taking aspirin to prevent heart disease.
The Harvard team identified 4,000 breast cancer patientsbetween 1976 and 2002 and followed them until their deaths or the endof the study in June 2006.
Altogether 341 women died from the cancer.
The Harvard study falls short of the research "goldstandard", however, because the women reported their aspirin use inquestionnaires, rather than going through a controlled clinical trial.
The next stage of drug development would normally be a large randomised study, but this may never happen.
Not only is aspirin so cheap it will not make any money for drugfirms, it could be hard to find a group of women with breast cancer whowere prepared never to take aspirin during a trial.
Nick Henderson of the European Aspirin Foundation, whichrepresents the industry, said: "The best evidence for aspirin inprotecting against cancer has been from studies on bowel cancer.
"But this latest report adds to growing evidence that aspirinappears to have special effects in reducing cancer risk through amechanism which has yet to be scientifically explained.
"However,I doubt it will be possible to carry out a proper clinical trialbecause everyone has heard aspirin may be useful. We"re going to hold aconference to plan the way forward."
Ed Yong, head of health information at Cancer Research UK,said: "Several studies have found that taking aspirin and related drugsis associated with a lower risk of breast cancer, and this new studysuggests that they might also help to stop cancer from spreading andimprove a woman"s chances of survival.
"But aspirin has risks as well as benefits, so we need largeclinical trials to see if it can really save lives from breast cancer,and, if so, to work out what doses to use and how long to use the drugsfor."