The Irish Prime Minister, John Bruton, said the fresh allegations - made by the unidentified paratrooper, who was on duty in Derry on the day of the killings in 1972 - were "very serious." The Fianna Fail opposition leader, Bertie Ahern, said it was "an inexplicable act of state terrorism".
The paratrooper told Channel 4 News that testimony for Lord Widgery's Tribunal of Inquiry into Bloody Sunday had been tailored to support a version of what had happened.
Mr Bruton, speaking from Washington, where he has been having Ulster peace process talks with President Bill Clinton, said the soldier's charges would be a key section of a dossier on Bloody Sunday being prepared by his government for submission to the British government.
"The evidence is very strong and very specific as to what happened and when. It is very important that it should be a central part of our dossier, and that the British government - as they have said they would - make urgent, objective assessment of it and take what ever steps are necessary to rectify the situation," he said.
The paratrooper told Channel 4 News: "Basically the Widgery tribunal - my experience of it was that you said what favoured the line they wished to take or you tended to be ignored. The officers present looked through my statement, removed it from the room and returned some time later with a second statement, which had various changes made to it.
"It wasn't exactly what I had previously said. In the event, I wasn't called forward to give evidence anyway."
He said of the civil rights march which ended in tragedy: "The situation became something of a shambles in every respect ... there was certainly no order to fire. The firing started in a spontaneous way."
Dick Spring, the Deputy Irish premier and Foreign Minister, said the soldier's claims were "very disturbing" and, together with other pieces of evidence that have come to light, provide a compelling case for a re-examination of the circumstances behind the killings.