Researchers from the US's National Toxicology Program didn't test the cancer risks of newer technologies, such as 4G and 5G or wi-fi, and so can't be sure if they have similar risks.
Although the radiation is definitely carcinogenic, people may have to be exposed to very high levels before they developed cancer, the researchers say.
They exposed laboratory rats and mice to levels that were up to four times greater than the permitted safety levels, and above that which humans would experience. The animals were exposed to 10-minute bursts for nine hours a day for two years; during that time, the rats developed cancerous tumours in their hearts, and others also had brain and adrenal gland cancers.
The researchers say the animals developed different cancers because their whole bodies were exposed to the radiation, but in humans this would most likely occur in the brain when a cell phone is placed close to the ear.
The study took 10 years to complete, and researchers say it was far more controlled and reliable than previous studies, which, in the main, had relied on questionnaires for accessing levels of exposure to the radiation.