The watches – which are often handed down as family heirlooms – give off harmful amounts of radon gas which can cause lung cancer.
Paint used to make the dials glow in the dark is the source of the gas.
Radon exposure is a leading cause of lung cancer deaths – and families are being warned not to open up the timepieces.
In a study by researchers at the University of Northampton and Kingston University, a collection of 30 antique radium-dial watches gave rise to radon concentrations 134 times greater than the recommended safe level when kept in a space the size of a typical box room.
Experts say that the levels are high enough to be dangerous in much larger spaces such as a whole house.
One of the report’s authors, Dr Robin Crockett said: “These results show that radon emitted from individual watches can potentially pose a serious cancer risk.
“This is of concern because in addition to military watches being particularly prized by collectors, many individual radium-dial watches are kept as mementoes by ex-servicemen and their descendants.
“They have the potential to pose a significant health hazard to themselves and their families.”
Professor Gavin Gillmore, of Kingston University, said that due to the age and condition of many of the watches in the study, opening them up without taking appropriate precautions is not advised.
He said: “Loose paint fragments will contain radium particles which could be ingested.
“As this is a strong alpha emitter there is a potentially serious health risk for those who do this.”
Radon is a colourless and odourless radioactive gas formed through the radioactive decay of small amounts of uranium that occur naturally in all rocks and soils and can seep into homes, where it can reach dangerous levels.
It is the second biggest cause of lung cancer deaths in the UK after smoking, with more than 1,100 fatalities every year.
Professor Gillmore added: “The watches tested were a mix of British, Swiss and American made items manufactured between the 1920s and 1960s, but we know these sort of paints were used in the 70s.
“There are potentially millions of these watches in circulation.”