An undercover British agent encouraged protesters to attack Irish police officers at an EU summit in Dublin, the Guardian newspaper has reported.

The undercover policeman Mark Kennedy did more than just observe and was an active participant in the proceedings, according to the newspaper.

The militant anti-capitalist protestors that were at the summit, which marked the accession of eastern European states to the union, are saying that Kennedy made visits to Dublin to help train protesters and encouraged other activists to attackthe police, raising further questions about his role as an undercover officer and supports claims he acted as an agent provocateur.

Michael D Higgins of the Irish Labour party is demanding an explanation from Britain as to why one of its undercover policemen was operating in the republic.

"This kind of activity is totally unacceptable … There are many of us who are familiar with the destructive consequences, in terms of democracy, that have flowed from this kind of activity from the 1970s on in relation to Northern Ireland," said Higgins.

Anti-capitalist activists believed Kennedy to be a fellow protester named Mark Stone during the violent clash between demonstrators and the Irish police riot squad in Dublin’s Phoenix Park on May Day 2004. The area around the park was sealed off by a large security cordon as police sought to protect the EU heads of states in Dublin for the ceremony at the Irish president's residence. While most of the demonstrators were peaceful, about 500 attacked the gardaí.

Said one of the protesters on Kennedy's actions on the day: "I saw him taking off his balaclava as he was coming out of the crowd. I was amazed that someone would stand close to police lines and take his mask off."

It was evident before the protest that police had intelligence on the activities of the activists who had travelled across the Irish Sea from Britain, suggesting that they had an informant in the protest movement. A police unit from the Pearse Street station arrested several people after smashing its way into a flat on Leeson Street in Dublin occupied by English anarchists.

"Some people arrested from the UK were kept here for two months but he did not get nicked for that. He was one of the people who were encouraging a confrontation with the garda up at the Parkgate," said an activist who played host to Kennedy.

He said Kennedy made at least two other visits to Ireland over the next two years, including acting as a trainer on a program on civil disobedience for anarchist activists in 2004 and two years later joining the mass demonstration against the visit of George Bush to Ireland.

"I found him very personable, very affable. I never suspected him. Growing up in the north of Ireland you know there are people out there who are informers but I never had any strong doubts about him," said the anarchist.