Martin McGuinness has been allowed to "hijack" the state visit to London of Irish President Michael D Higgins, Ulster Unionist leader Mike Nesbitt has claimed.
But, as he attended the latest event in the Guildhall in London last night, the UU chief added that as a result Sinn Fein would now have to consider dropping its historic House of Commons boycott.
Mr Nesbitt added that there was a lot of bad feeling among unionists and others that Mr McGuinness had been able to "hijack" the visit of the Irish President.
"It was not about Britain and Sinn Fein but about Britain and Ireland and that was overshadowed to some extent by McGuinness taking part and some people are very cross about that," he said.
"The President's speech at Windsor Castle was particularly good on dealing with the past but to a certain degree Sinn Fein have hijacked that.
"But I think they will now have to consider dropping the House of Commons ban. I have heard Sinn Fein members explain they have offices in London and just do not go into the chamber. Sooner or later it will be the same as the Assembly, where they have offices and the chamber.
Former Sinn Fein press officer Danny Morrison insisted, however: "I don't see that happening at all.
"If you end abstentionism in the British Parliament how then could you credibly complain about Britain legislating and interfering in Irish political affairs?"
Mr Morrison said he did not think Mr McGuinness had "put a foot wrong" during the state dinner which the deputy First Minister attended at Windsor Castle on Tuesday. The former IRA commander observed protocol by shaking hands with the Queen and raising a toast to her.
"I think the republican base is very mature and pragmatic and understands the issues at several different levels," Mr Morrison went on.
Meanwhile, Sinn Fein leader Gerry Adams has angrily accused Lord Tebbit of publicly encouraging the assassination of Martin McGuinness.
Responding to the former Conservative chairman's controversial remarks, Mr Adams said it was unacceptable for a member of the House of the Lords to openly support the shooting of a political leader.
"I fully understand that Norman Tebbit has himself been a victim of the political conflict and I regret that he has suffered grievously," said Mr Adams.
"However, to publicly advocate the assassination of deputy First Minister Martin McGuinness is a shocking throwback to a violent past from which we are seeking to move on."
Mr Adams said his party colleague has worked tirelessly to ensure the success of the peace process.
"Martin, his home and indeed his family have been targets for abuse and attack by so-called dissident republicans," he said.
Outspoken Lord Tebbit, who was injured with his wife during the 1984 IRA bombing of the Grand Hotel in Brighton, said he hoped Mr McGuinness is shot in the back for attending the State banquet.
Mr McGuinness said the remarks were "not fitting".