The common painkiller aspirin could help prevent cancers spreading by boosting the immune system so it destroys tumour cells, a study found. Researchers had previously observed that patients taking a daily low dose of aspirin seemed to be less likely to see their disease spread, but the reason why was not understood.

Now, Cambridge University scientists have described the “Eureka moment” when they discovered the answer may lie in the drug’s effect on immune cells. Their work, which involved genetic analysis of mice, found that aspirin cuts production of a clotting factor which suppresses T cells. Those cells are responsible for identifying and attacking cancerous cells.

Researcher Dr Jie Yang said: “It was a Eureka moment when we found [the clotting factor] TXA2 was the molecular signal that activates this suppressive effect on T cells.

“Before this, we had not been aware of the implication of our findings in understanding the anti-metastatic activity of aspirin. It was an entirely unexpected finding.”

Dr Yang added that aspirin, or other drugs targeting the same pathway, may be cheaper and more widely accessible than more complex cancer therapies.

However, the scientists stressed that aspirin can have serious side effects and people should consult their doctor before taking it.

Study leader Professor Rahul Roychoudhuri, also from Cambridge, said: “Despite advances in cancer treatment, many patients with early stage cancers receive treatments, such as surgical removal of the tumour, which have the potential to be curative, but later relapse due to the eventual growth of micrometastases – cancer cells that have seeded other parts of the body but remain in a latent state.

“Most immunotherapies are developed to treat patients with established metastatic cancer, but when cancer first spreads there’s a unique therapeutic window of opportunity when cancer cells are particularly vulnerable to immune attack.

“We hope that therapies that target this window of vulnerability will have tremendous scope in preventing recurrence in patients with early cancer at risk of recurrence.”

Researchers are now working with Professor Ruth Langley at University College London, who is leading the Add-Aspirin clinical trial, to find out if aspirin can stop or delay early stage cancers from coming back.

That trial, supported by Cancer Research UK, includes patients with breast, oesophageal, stomach, prostate and bowel cancer.

Professor Langley said of findings, published in the journal Nature: “This is an important discovery. It will enable us to interpret the results of ongoing clinical trials and work out who is most likely to benefit from aspirin after a cancer diagnosis.

“In a small proportion of people, aspirin can cause serious side-effects, including bleeding or stomach ulcers.

“Therefore, it is important to understand which people with cancer are likely to benefit, and always talk to your doctor before starting aspirin.”