One of the phones in the test had such high levels of bacteria that it could have given its owner a serious stomach upset.
The findings from a sample of dozens of phones by Which? magazine suggest 14.7 million of the 63 million mobiles in use in Britain today could be potential health hazards, reports the Daily Mail.
Hygiene expert Jim Francis, who carried out the tests, said, "The levels of potentially harmful bacteria on one mobile were off the scale. That phone needs sterilising."
The most unhygienic phone also had 39 times the safe level of enterobacteria, a group of bacteria that live in the lower intestines of humans and animals and include bugs such as salmonella.
It boasted 170 times the acceptable level of faecal coliforms, which are associated with human waste.
Other bacteria including food poisoning bugs e.coli and staphylococcus aureus were found on the phones but at safe levels.
Which? researcher Ceri Stanaway said, "The bugs can end up on your hands which is a breeding ground and be passed back to your phone. They can be transferred back and forth and eventually you could catch something nasty.
"What this shows is how easy it is to come into contact with bacteria. People see toilet flushes as being something dirty to touch but they have less bacteria than phones.
"People need to be mindful of that by observing good hygiene themselves and among others who they pass the phone to when looking at photos, for example."
Which? has previously found that some computer keyboards carry more harmful bacteria than a lavatory seat.