Emergency and security services were put to the test to see if the nuclear power station could handle a major incident.

The exercise, which took place towards the end of last year but was only disclosed at this week’s meeting of the Sizewell Stakeholder Group (SSG), was instigated by the Office for Nuclear Regulation (ONR), the UK nuclear safety and security agency, in conjunction with the Civil Nuclear Constabulary (CNC).

The CNC has a team of armed officers working on and off the nuclear site to deter or detect and respond to terrorist activity.

However, despite a plea for more transparency, the scenario for the “terrorist attack” is being kept secret for security reasons although the ONR described it as “credible, challenging and well planned”.

An ONR report said the agency’s security inspectors had observed the exercise.

It said: “It was considered that the exercise was an adequate demonstration of the arrangements. The exercise generated a number of learning points. In addition the ONR observers also noted several good practices

“Areas for further improvement will be monitored by the ONR security inspectors for progress during future inspections.”

The exercise was well supported by external agencies thought to include Suffolk emergency services and Government security experts.

Graham Moorcroft, ONR’s Sizewell B inspector, said in response to a request from SSG member Pete Wilkinson for more information: “It is difficult to report on security matters in a public forum.”

But Mr Wilkinson said the ONR should provide more transparency in order to increase public confidence in measures to deter or respond to a potential terrorist threat.

“There is nothing to stop you getting two or three SSG members, clearing them for security purposes and let them see what is going on,” he said. “It is also time you demonstrated that an evacuation of the area can work. Otherwise you have to accept it is a nonsense.”

Mr Moorcroft said the ONR had concerns over testing public evacuation, partly because of practicality but also cost.