Legal action is being taken by more than 200 holidaymakers amid claims of undercooked food, sewage smells and a dirty swimming pool at a Butlins resort.
Lawyers say "worrying accounts" have emerged over the cleanliness and standards of food preparation at Butlins in Bognor Regis, West Sussex.
Law firm Irwin Mitchell said it is representing more than 215 holidaymakers in a legal action over allegedly ruined holidays in 2011.
A Butlins spokesman said it would "robustly defend" the "speculative action" and that its cleanliness record was recognised worldwide in the tourist industry.
Irwin Mitchell said legal action was the only option left for the holidaymakers following a denial of liability by Butlins operator Bourne Leisure.
Elizabeth Tetzner, representing the group, said: "Our clients have given us worrying accounts of their stays, including concerns regarding cleanliness, standards of food preparation and service as well as their treatment by staff at the site."
But a spokesman for Butlins at Bognor Regis said: "We will robustly defend this speculative action raised at the High Court in Birmingham.
"We operate our resorts to the highest health and safety standards and work closely with the Health and Safety Executive to ensure that our guests' safety takes priority.
"Our standards of cleanliness and hygiene are recognised throughout the tourist industry worldwide.
"Such is our reputation on health issues that environmental officers from around the UK have described our policies and procedure as outstanding."