Campaigners are delivering 4,000 leaflets warning residents of what they claim is the potential grave danger of Sizewell B following changes announced to its emergency zone.

Together Against Sizewell C (TASC) is angry over the proposed reduction of the area where potassium iodate tablets will be issued – only to those living one kilometre from the nuclear power station – and says the evacuation zone is “ridiculously small”.

However, Sizewell’s owner EDF Energy says the risk of an accident has significantly decreased since Sizewell A stopped operating eight years ago, and the chance of an accident at Sizewell B requiring the off-site emergency plan to swing into operation was “once in every 300,000 years”.

A comprehensive review of emergency planning has been carried out and the Office for Nuclear Regulation (ONR) has decided to dispense with its circular 2.4kilometre zone and to replace it with one based largely on postcodes.

This will mean that parts of the emergency plan zone will stretch up to around 3km from the power stations and both Leiston and the majority of nearby Aldringham will in future be included in it.

People living in this zone will be alerted to any serious incident at Sizewell and be updated with information and advice on taking shelter or evacuation, while those within one km will be issued with potassium iodate tablets (stable iodine) as a counter-measure.

Jeremy Western, director of special projects and nuclear new build for EDF, said Sizewell B was the most modern and safe nuclear reactor in the world and it was “exceedingly unlikely” it would ever have an accident that breached the station’s boundaries, let alone anything worse, and the risk of an incident at the site was now less than previously.

The coastal station’s safety systems feature a series of barriers to withstand various incidents that might occur and the ONR – which considers reasonably foreseeable accidents – is satisfied that the stringent on-site precautions, measures and actions in place are more than adequate.

Even though a nuclear emergency plan has never had to be used in the UK, a new multi-million pound emergency response centre also opened this year at Sizewell to increase safety.

Mr Western said: “A reasonably foreseeable accident, in nuclear safety terms, is a very, very unlikely event – we are talking about events that could happen once in every 300,000 years statistically.”

TASC, which is leafleting Leiston, believes the emergency zone should be much wider and says after the Fukushima disaster, the Japanese authorities imposed an exclusion zone of 20kms.

Pete Wilkinson, acting chairman of TASC, said: “It is time we took the lid off this debate and told people exactly how the authorities are gambling with their lives, their livelihoods, their jobs, their homes and farmland.”

Some experts claimed nuclear accidents of a severe nature were likely every 10 to 20 years and “yet our authorities seem hell-bent on keeping the evacuation zone at a ridiculously small radius – only 23 individuals would be affected by a 1km radius evacuation zone at Sizewell – so as not to frighten the second homers, tourists and business investments in the area”.

He added: “It is our view that we all have a right to know the truth and that the authorities have a duty to provide unbiased and accurate information to a wide constituency of people.

“If the proposed 1km evacuation zone is agreed, it will not be with the consent of residents, nor with the agreement of experts outside the industry, because they are excluded from the discussions.”