A study of three hospitals found contamination was "significantly higher" in toilets on the days jet air dryers were used compared to paper towels.
Jet air hand dryers in hospital toilets spread more germs than disposable paper towels and should not be used, scientists have advised.
Writing in the Journal of Hospital Infection, researchers argued that official guidance about how to prevent bacterial contamination in hospital buildings needs to be strengthened.
A study carried out in three hospitals found contamination - including by faecal and antibiotic-resistant bacteria - was "significantly higher" in toilets on the days that jet air dryers were in use compared to paper towels.
Scientists said the problem lies in people not washing their hands properly - meaning microbes get blown off by jet air dryers and spread around the room.
They said official Department of Health guidance is that air dryers can be placed in toilets in the public areas of a hospital, but not in clinical areas - but this is not because of the risks they pose for cross contamination, but because they are noisy.
Mark Wilcox, professor of medical microbiology at the University of Leeds - who supervised the international study, said the guidance needs to focus on the infection risks given the new evidence.
The study looked at bacterial spread in a real world setting - in two toilets in each of three hospitals, which were in the UK, France and Italy.
Each of the toilets had paper towel dispensers and jet air dryers, but only one of these was in use on any given day.
On each day, over 12 weeks, levels of bacterial contamination in the toilets were measured, allowing comparisons to be made when either paper towels or jet air dryers were in use.
Samples were taken from the floors, air and surfaces in each of the toilets.
The main target bacteria were Enterobacteria including Escherichia coli or E.coli, which cause a wide range of infections including gastroenteritis, pneumonia and septicaemia, and enterococci - bacteria that can cause difficulty when treating infections.
They also looked for Staphylococcus aureus, which is responsible for a range of conditions, from minor skin and wound infections to life-threatening septicaemia.
Across the three hospitals, bacterial counts were significantly higher in the toilets on the days that jet air dryers were in use.
The study is the largest of its type to investigate whether the way people dry their hands has an impact on the spread of bacteria.