Squirting the murky water that builds up inside these toys could expose children and adults to dangerous bacteria, the researchers said.
Some 80 per cent of bath toys carry dangerous bacteria
Researchers carried out controlled experiments on toys over 11 weeks, simulating typical bathtimes.
They exposed the toys to dirty bath water, soap, sweat and urine.
In addition, they also collected bath toys from real households as part of the experiment.
All the bath toys were cut open and analysed in the laboratory.
Some 80 per cent of all the studied toys contained potentially dangerous bacteria.
As well as Legionella, the researchers found Pseudonomas aeruginosa; a drug-resistant 'superbug' that can be deadly, particularly in people with weak immune systems.
Plastic nourishes bacteria
Researchers from Swiss Federal Institute of Aquatic Science and Technology found between five million and 75 million bacterial cells per square centimetre on the inner surfaces.
The researchers found that the plastic helped nourish the bugs - releasing carbon compounds they can eat.
Other key nutrients such as nitrogen and phosphorous, and additional bacteria, were contributed by the human body via bodily fluids such as sweat and urine, as well as external pollutants and personal care products.
Dr Frederik Hammes, one of the authors, said he was not surprised by the findings: "Mouldy bath toys are widely discussed in online forums and blogs, but they have received little scientific attention to date."
He said the bath toys are extremely interesting for researchers - particularly highlighting how plastic items can be an incubator or bacteria - posing a risk to the vulnerable, particularly children who may enjoy squirting water from bath toys into their faces.
Dr Hammes comments: "This could strengthen the immune system, which could be positive, but it can also result in eye, ear, or even gastrointestinal infections."
Experts say that options for parents concerned could range from banning bath toys altogether, or less drastically, avoid toys that allow filthy water to accumulate inside.
Should children play with second-hand plastic toys?
Old plastic toys, including lego, dinosaur figurines and dolls, can contain dangerous levels of toxic chemicals, the first study of its kind suggested in January 2018.
Out of a sample of 200 trinkets found in homes, nurseries, schools and charity shops, more than 10 per cent contained high levels of hazardous elements that can be toxic at low amounts and would not be approved for sale today.
Children are thought to be more at risk of poisoning from hazardous elements as their organs are still developing and they are more likely to put non-edible objects in their mouths.
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Lead author Dr Andrew Turner from the University of Plymouth, said: 'Second-hand toys are an attractive option to families because they can be inherited directly from friends or relatives or obtained cheaply and readily from charity stores, flea markets and the internet.
"The attractive cost, convenience and recyclability of previously used toys has the potential to create a legacy of chemical contamination for younger children."
Since 1995 toys must comply with the Toy Safety Directive, which includes safe hazardous element levels, to be sold in the EU. There is no regulation regarding the recycling or re-sale of old toys.
The researchers analysed toys found across the south-west of England.
Such toys were made of plastic and were small enough to fit in young children's mouths; or were made up of removal parts.
The researchers used specialist X-ray fluorescence scans to detect levels of hazardous elements.
