Adam Busby, 50, the self-styled leader of the Scottish National Liberation Army, was arrested by Irish detectives at his home in Dublin at dawn on Saturday following a month-long joint investigation by Scotland Yard and the Gardai.
The inquiry was launched after letters from a group calling itself the Republican Revenge Group were sent to the British Ambassador and the Papal Nuncio in Dublin, and Ofwat, the water industry regulator in Birmingham. A fourth letter is thought to have been sent to Bertie Ahern, the Irish Prime Minister, but was never traced.
The letters threatened to contaminate the English water supply by introducing a common weedkiller through the network of water hydrants unless there was a commitment to a "total British military and political withdrawal" from Northern Ireland by 16 June.
The threats sparked a joint Anglo-Irish investigation sanctioned at the highest level. The Prime Minister, Tony Blair, chaired an emergency meeting of ministers to co-ordinate the inquiry which involved senior figures from the water industry, toxicology experts and the security services.
Experts said the risk to the public was "negligible" but fears were expressed for kidney dialysis patients who were most vulnerable to water contamination.
Newspaper editors in the UK and the Irish Republic were briefed two weeks ago.
They agreed to a news blackout in order not to fuel public hysteria that might hamper an investigation or give the blackmailer the publicity demanded in the letters.
Last night senior detectives defended the news blackout and played down the risks but admitted that surveillance of the country's million or so water hydrants would have proved impracticable.
David Veness, assistant commissioner at Scotland Yard,said the threat letters pointed to a calculating and intelligent individual who was acting in a controlled and deliberative, rather than vicious or psychotic, manner.
The man arrested on Saturday was detained under the Irish Republic's anti-terror legislation. He can be held until tomorrow without charge.Reuse content