A leading East Anglian anti-nuclear campaigner was interviewed under caution by police after a "suspicious" substance was found in an envelope sent to the offices of the Nuclear Decommissioning Authority in Cumbria.

A leading East Anglian anti-nuclear campaigner was interviewed under caution by police after a "suspicious" substance was found in an envelope sent to the offices of the Nuclear Decommissioning Authority in Cumbria.

Charles Barnett, who categorically denies sending the substance, went to Leiston police station to be questioned by detectives after white powder was found in an envelope he had addressed to the NDA.

The member of staff who opened the package immediately alerted security personnel and the area was evacuated in line with emergency procedures. After carrying out tests, police declared the substance to be harmless, and it is thought to have been sugar. Detectives are treating the incident as an attempt to cause fear and have launched a full criminal investigation.

Mr Barnett is chairman of the Shut Down Sizewell Campaign, which has been strongly critical of the NDA in its refusal to order the Sizewell Stakeholder Group, the local nuclear site liaison group, to include representatives of anti-nuclear organisations and to open its sub-committee meetings to the public.

But he said any kind of criminal act would be contrary to the group's aims.

Mr Barnett said he sent the envelope to the NDA with copies of his group's latest newsletter.

"It had my hand-writing on it, my address was on the back and a compliments slip was inside. I have no idea how the sugar got into the envelope. I have looked at a photograph of it, showed to me by the police officer, and can see no sign of it being tampered with. It is a mystery. We have always been a non-violent group and any suggestion that any of us would be involved in this kind of thing would be ridiculous," he said.

He attended Leiston police station on Thursday for a tape-recorded interview. It is understood a copy of the tape is being sent to Cumbria police who will decide whether further action is needed.