Lorry driver Andy Toms had taken his step daughter Amie, 11, and a friend for a swim at Littledown leisure centre in Bournemouth, but was informed he could not enter the water because of his inappropriate attire.
Andy, 45, was told his knee-length trunks were a health and safety risk to other swimmers.
The furious dad-of-six left the complex and got a refund - and said that he will never use the pool again: "I left and had to take the two kids with me because at 11 and 13, they were too young to be left without an adult," he said, "We had planned the trip the night before, so they weren't very happy.
When I asked to speak to a manager, all they said was that it was company policy. It's totally ridiculous. I was furious.
Andy says he has used the pool for years and worn his three-quarter-length shorts before. The leisure centre claimed the trunks were not swimwear, and could introduce bacteria in to the water.
Andy told The Sun: "They offered a pair from lost property but if the problem's hygiene it's not very hygienic to wear somebody's lost trunks."
A spokesman for the leisure centre said: "These are not classed as swimwear and may have been used for other things like running in a field, which could mean bacteria going into the pool."
Have you ever been thrown out of a pool for having the wrong type of cossie?
Or do you agree that 'long shorts' should not be allowed?