Scientists have identified a gene they believe plays a major role in more than half of all breast cancers and a significant portion of other tumours.

The gene, which helps to stop cancer cells in their tracks, came to light after researchers noticed it was missing from tissues that had been removed from breast cancers for testing. The lack of the gene has also been implicated in half of all cases of colon and prostate cancer, and a quarter of ovarian and bladder tumours.

Cancer charities described the discovery as a "major step forward" that could open up new ways to screen for and treat the disease.

Paul Edwards, a molecular biologist at Cambridge University, said the discovery could be the most important cancer-suppressing gene of the past 20 years. "This is a gene lost in a quarter to a half of common cancers, so it is clearly playing a really important role," Edwards said.

Scientists have long known that cancerous tissues are often missing fragments of chromosomes, the tightly coiled packages of DNA that carry our genes.

Edwards and his colleagues looked at tissues from 54 breast tumours and found part of chromosome 8 was missing in more than half of them. After cross-checking against the Human Genome Project they were able to identify a gene called NRG1 that was lost.

"In every case we looked at where a big chunk of chromosome 8 had been lost, at least part of the gene was lost," said Edwards, whose research is published in the journal Oncogene. When the gene is missing, healthy tissues lose their ability to guard against cancer cells, he said.

Researchers are still working out how chromosomes become broken in the first place, but by identifying the gene, they hope to develop new ways of screening people who are susceptible to cancer and possibly even finding new treatments.

Further experiments showed that blocking the effect of NRG1 in healthy breast cells made them divide more quickly. The research was funded by Breast Cancer Campaign and Cancer Research UK.

The discovery of NRG1 is thought to be the most significant step forward in the field since another gene, p53, was discovered in the 1970s and found to be implicated in cancers in the late 1980s. The gene was the first "tumour suppressor" gene found in cells and is known to be faulty or inactivated in many types of the disease.

"Knowing the identity of this gene will lead to far more detailed studies of how it works and how it is involved in breast cancer development," said Arlene Wilkie, director of research and policy at Breast Cancer Campaign.

"In the UK, 12,000 women die from this disease every year, so it is vital we understand how breast cancer develops in order to stop it happening."

Lesley Walker, director of cancer information at Cancer Research UK, said: "This discovery is an important step forward in understanding a disease that more than 45,500 women are diagnosed with in the UK each year. More research is now needed to understand how this 'guard' gene is silenced and how exactly this influences the development of cancer. It might then be possible to develop ways to bypass the gene or target treatments to the defect," she added.