Chris Davies attacked Nigel Farage's party for calling for the scrapping of an EU bathing water directive.
Paul Nuttall, Ukip's Deputy Leader, had launched an attack on the new standards, claiming they could jeopardise tourism on British beaches.
But Davies, the North West MEP who is the Lib Dem spokesman on the environment in Brussels, described Nuttall's claims as "bullshit" in a statement on his website.
In the posting, called "Let them eat shit, says UKIP's Paul Nuttall", Davies said millions of holiday-makers would be more likely to fall ill if the directive was scrapped.
He said: "If UKIP get their way British travellers can expect to experience the shits a great deal more often.
"Contaminated sea water is a major cause of vomiting and diarrhoea, and people should have a right to know about water quality.”
He claimed the dictat had driven up water quality standards across Europe, adding: "By keeping people in the dark, UKIP will keep holiday makers confined to their bathrooms instead of enjoying themselves on the beach."