Mouth cancer can develop in two specific ways, researchers have found.
Smoking is a major cause of mouth cancers
Doctors had previously understood that all cases of mouth cancer developed in the same way.
The work by a team at Glasgow's Beatson Institute for Cancer Research provides clues about how the different forms of cancer could be detected early.
The findings, published in the journal Cancer Research, may also help identify patients with the more aggressive form of the disease.
Around 1,600 people die each year in the UK from mouth cancer.
The most common indicators of mouth cancer are persistent sores, ulcers, red or white patches and unexplained pain in the mouth or ear.
People can also have lumps in their necks, a persistent sore throat or difficulty swallowing.
In this study, researchers took samples from the mouths of 19 people with pre-cancerous lesions, or spots, 16 patients with mouth cancer and the mouths of four healthy people.
They then compared the samples, which each contained thousands of cells.
They found two distinct genetic patterns in the samples from patients with cancers and lesions.
In those with the most aggressive form of tumour, faults were found in the p53 gene, which should act to stop damaged cells dividing and so stop cancers growing, and no expression of the p16 gene which controls cell regulation.
Cells in this form of mouth cancer were dubbed "immortal", because they keep on dividing, making them more likely to spread and to cause a recurrence.
"Mortal" cells - found in the second kind of tumour - have a limited lifespan and so will exhaust themselves as they develop into a tumour, and so are less likely to spread or recur following treatment.
Professor Paul Harrison, who led the research, said: "We found that many of the molecular changes found in 'mortal' and 'immortal' cancers are also found in their respective pre-cancerous lesions, which suggests that mouth cancer forms in different ways."
"The data we collected provide strong evidence for the first time that some mouth cancer tumours are more aggressive than others and are therefore linked to poorer patient survival.
"We hope in the future that these findings will allow us to discover early on who needs urgent treatment and possibly offer new methods of preventing the disease."
Professor John Toy, medical director at Cancer Research UK, said: "Cases of mouth cancer in the UK have risen by a quarter over the past 10 years so these are valuable findings that will help scientists gain a clearer understanding the ways the disease can develop and progress."