The parents have refused to let their children have the MMR vaccine due to worries about its side-effects. They believe the best immunisation is to deliberately expose them to childhood diseases.

But the action has been criticised by Dr Meirion Evans, consultant in communicable diseases for the Bro Taf health authority..

He said the parents were acting irresponsibly.

Critical point

"It defeats the whole point of having a vaccination which is to build up artificial immunity and prevent the disease," he said.

"What's more there are far more potential complications from having the disease than having the vaccination."

"Wales is at a critical point with MMR vaccinations- we have gone from a high of 95% down to an average of 86/87%.

"But in certain parts of west Wales in particular it is 80% - so we are on the threshold of the disease coming back if we are not careful.