Anti-nuclear campaigners are seeking to halt construction of a new next-generation nuclear reactor on the Cotentin peninsula, claiming the site is breaching planning rules and putting people’s health at risk.

Anti-nuclear group Crilan has written to the French nuclear safety authority calling for the construction work to be blocked at Flamanville, on the Channel coast near Cherbourg.

Activists say changes made to the design of the cladding around the reactor core present a safety risk and they claim the changes have also invalidated the original planning permit. EDF is preparing a safety report on the Flamanville project that will be filed with the nuclear safety regulator by the end of this year. Its contents will determine whether the reactor is allowed to start working or whether more modifications will be ordered.

The company said it would be inappropriate to comment on safety at Flamanville until the report had been compiled and submitted, but added in a statement: “Nuclear safety is EDF’s top priority.”

The group has had to announce that the European Pressurised Reactor (EPR) is running two years behind schedule and will cost €1bn more than first thought, with the final bill estimated at €5bn.

The plant is now due to begin producing electricity for Normandy and Pays de la Loire households in 2014.

The reactor is considered key for the future development of the French nuclear industry. It is seen by EDF as a prototype and it hopes to sell similar reactors to other countries.

If all goes to plan, the town of Penly, near Dieppe, has been earmarked as the second French site for an EPR reactor.