Thousands of cancer patients could have their disease detected earlier every year thanks to scientists identifying a new "risk marker" for cancer.
Researchers have found that patients who have a high blood platelet count - which can be identified through a simple blood test - are significantly more likely to go on to develop cancer than those who do not.
Some of these patients have a raised blood platelet count, known as thrombocytosis, before they even suffer symptoms of cancer.
Showing a high platelet count could prompt early investigations, researchers said.
And detecting cancer sooner means that patients have better chances of survival.
The University of Exeter Medical School said that the study has revealed the first new indicator of cancer to have been robustly identified in 30 years.
Researchers from the university said that thrombocytosis has recently been recognised to have diagnostic potential. But no study has looked at GP data on whether having a high platelet count leads to cancer diagnosis of any sort.
The authors looked at data for 40,000 patients using the UK Clinical Practice Research Datalink - which holds anonymised patient records from 8% of UK GP practices - and linked information from the English Cancer Registry.
The study, published in the British Journal of General Practice, looked at data for patients over the age of 40 who had a primary care full blood count taken.
They found that 11.6% of men and 6.2% of women with thrombocytosis went on to be diagnosed with cancer within a year.
This rose to 18.1% of men and 10.1% of women if a second raised platelet count was recorded within six months.
As a result, GPs should consider further investigations into possible cancer if a patient has a high blood platelet count, they concluded.
Patients with lung or bowel cancer were more commonly diagnosed with thrombocytosis - and a third of these patients showed no other symptoms indicative of cancer.
The researchers calculated that 5% of patients with cancer have thrombocytosis before a cancer diagnosis. One third of them have the potential to have their diagnosis expedited by at least three months by the identification of this risk marker.
This equates to 5,500 earlier diagnoses annually across the UK.
"We know that early diagnosis is absolutely key in whether people survive cancer," said lead author Dr Sarah Bailey, of the University of Exeter Medical School.
"Our research suggests that substantial numbers of people could have their cancer diagnosed up to three months earlier if thrombocytosis prompted investigation for cancer.
"This time could make a vital difference in achieving earlier diagnosis."
Professor Willie Hamilton, of the University of Exeter Medical School, added: "The UK lags well behind other developed countries on early cancer diagnosis.
"In 2014, 163,000 people died of cancer in this country.
"Our findings on thrombocytosis show a strong association with cancer, particularly in men – far stronger than that of a breast lump for breast cancer in women.
"It is now crucial that we roll out cancer investigation of thrombocytosis. It could save hundreds of lives each year."