Unionists are furious that the Northern Ireland Secretary, Mo Mowlam, asked the pair to attend the event at Hillsborough Castle in County Down next week.

Prince Charles's uncle was killed by the IRA

Prince Charles's uncle, Lord Mountbatten, was murdered by the IRA in 1979. Unionists say the IRA and Sinn Fein are closely linked although Sinn Fein denies this.

A Northern Ireland Office spokesman confirmed that all of the province's 18 MPs have been asked to the party, the main social event of the year.

It is the first time that members of Sinn Fein have been invited. They were not on the list last year because the IRA had not declared a ceasefire.

Sinn Fein said its leaders would not go because of the Prince's links to the Parachute Regiment.

Prince Charles meets Parachute Regiment veterans

Party chairman Mitchell McLaughlin said: "Due to the circumstances surrounding the murders in Derry on Bloody Sunday and the presence of the Colonel in Chief of the regiment responsible, Sinn Fein will not be attending the garden party at Hillsborough."

Ms Mowlam defended the decision, saying Mr Adams and Mr McGuinness would not come face-to-face with any royals.

Royals have suffered too

She told BBC 1's Breakfast with Frost: "We will be very careful and we have organised it such that if they come ... it will be I that will escort the royal round and I will make very sure who he meets.

Mo Mowlam: "I would not put the Prince in that position."

"They haven't made it clear to me - the Royal Family - but I would not have put them at the garden party in a position where they would meet Sinn Fein."

She said she had met too many families of victims and said that the Royal Family, with the murder of Lord Mountbatten, "are a family that have suffered and I wouldn't put them in that position".

Asked if the Prince could shake hands with the Sinn Fein MPs, she insisted: "That won't happen. I can guarantee that."

The garden party had been structured so that the Royal Family would not be put in a "painful" position, she insisted.

'A foolish decision'

The Sinn Fein leaders' decision to decline the invitation did not dampen Ulster Unionist Party leader David Trimble's anger.

David Trimble: "A foolish and insensitive move"

He said: "I think the secretary of state will be relieved at their absence, particularly after the republican-orchestrated violence in Portadown.

"There's only one paramilitary organisation in that location who could have created those blast bombs and this is tantamount to a breach of the IRA ceasefire.

"I think it was remarkably foolish on her part to do this - and somewhat insensitive - when you consider that the republican movement murdered the Prince's uncle and was at one stage planning to murder him."

He said the only time when they could be invited would be if they ceased to be terrorist leaders and became democratic leaders.

'Embarrassing situation'

The Conservative Northern Ireland spokesman, Andrew Mackay, said: "I think this is a decidedly premature invitation."

"I am obviously pleased that the ceasefire has been sustained and that Sinn Fein/IRA appear to have properly signed up to the Belfast agreement.

"But there is still much for them to do.

"I fear that the government and the prime minister have put Prince Charles into an invidious and potentially embarrassing situation, and I regret that."