Northern Ireland Secretary Mo Mowlam faced Unionist anger last night after her invitation to Mr Gerry Adams and Mr Martin McGuinness to a royal garden party with the Prince of Wales was bluntly rejected by Sinn Fein.

She stood accused of an error of judgment and of using the monarchy for political purposes over her decision to include Sinn Fein's two MPs and several councillors on the guest list at her official Northern Ireland residence this week.

Up to 1000 people have been invited to Wednesday's garden party to mark the 50th anniversary of the National Health Service. The principal guest will be Prince Charles, whose great-uncle Lord Mountbatten was murdered by the IRA.

However, Sinn Fein said none of its representatives would attend because Prince Charles was colonel-in-chief of the Parachute Regiment, which was blamed for the Bloody Sunday massacre in Londonderry in 1972.

UK Unionist MP Robert McCartney said the invitation would have placed the prince in an ''invidious and unacceptable position''. He said: ''To attempt to use the monarchy in this manner for political ends is nothing short of despicable.''

Ms Mowlam initially was forced to pledge she would spend the party making sure Prince Charles did not come across the Republican leaders. ''I have met too many families of victims and the royal family, in that sense with the death of Mountbatten, are a family that have suffered and I wouldn't put them in that position,'' she said.

The republican rejection of the invitations saved the prince and the Northern Ireland Secretary any embarrassment, according to Shadow Ulster Secretary Andrew Mackay. But he feared the invitations could encourage unionists to elect people opposed to the peace agreement.

He said: ''This simple truth is that an invitation like this puts the fear of God into the unionist community. And the danger is they will react by actually electing unionist politicians who will be there to wreck or disrupt the assembly, which will not be in the interests of a lasting settlement.''

The row came in the aftermath of Saturday's rioting in Belfast, which saw Republican activists hurl petrol bombs at the RUC, injuring 15 officers.

Police responded with rubber bullets in the clash, which erupted as police guarded the route of a march by junior Orangemen near the nationalist Garvaghy Road area. The march was agreed by the Parades Commission. Four civilians were injured. Ulster Unionist leader David Trimble, member for Upper Bann, the area's MP, said: ''I am not surprised the republican movement is trying to use the tensions in the community trying to derail the peace process.''

qLabour MP George Foulkes told yesterday how he stressed the Catch 22 situation faced by his constituent, jailed Scots Guardsman Jim Fisher, of Ayr, and his fellow prisoner, Mark Wright, of Arbroath, when he met Ms Mowlam last week. ''I stressed to her that pictures of terrorists being released under the Peace Process amnesty could create difficulties if the two guardsmen are still in prison.

''The point I pushed to her was the prospect of seeing pictures of people celebrating their release who had committed crimes at least as bad and in some cases worse than Fisher and Wright while at the same time the two Scots cannot apply for amnesty because they are not terrorists.''

Both men were given life sentences when they were found guilty of killing teenager Peter McBride as he fled when they stopped him while on foot patrol in 1992. Both claimed they suspected he was carrying a bomb.

The MP added: ''I made it clear to Ms Mowlam I was not challenging the court verdict. What I am saying is that they have been six years in jail. The Peace Process is under way and we are going to see pictures of terrorists being released. Therefore, some consideration needs to be given to their position.''