A month-long joint police operation brought the arrest of a man in Dublin early today after three warning letters had been sent to senior figures, including British Prime Minister Tony Blair, press reports said. Irish police would only confirm that they had arrested a man in his 50S under a section of the Irish anti-terrorist act.

But security sources said that there had been a threat to commit mass murder by poisoning England's water supply from a group calling itself the Republican Revenge Group. The group demanded that British troops and politicians leave Northern Ireland by June 16, the sources said.

According to the sources, threatening letters had been sent to the British ambassador in Dublin, and to the Central England offices of the water regulatory body in Britain. Meanwhile, an IRA bomber releasedfrom jail after trying to blow up former British Premier Margaret Thatcher has threatened to kill the leader of the guerrillas' political wing Sinn Fein if he accepts the peace deal, a Sunday newspaper reported.

Patrick Magee, dubbed the `Brighton Bomber' after he exploded a bomb in an English seaside hotel where Thatcher and her ministers were staying, said he would kill Sinn Fein president Gerry Adams if he ``surrendered'' to the peace process, the Sunday Mirror reported.