Professor Anne Duggan has claimed that King Henry II, who invaded Ireland in 1171 under the pretence of having secured papal support for the incursion, purposely edited the pope's decree in order to make the invasion have the appearance of a just crusade.

According to Duggan, three key paragraphs that specifically forbade the invasion were removed from the document before the decree was brought to Ireland.

Duggan was speaking on a BBC radio programme entitled A Laudable Invasion, which likened the editing of the 12th century papal decree to the controversial 'sexed-up' document used by Tony Blair's government to gain support for the invasion of Iraq three years ago.

In order to ease his troops into Ireland, Henry II gained a papal decree . . . the infamous Laudabiliter . . . giving the Vatican's support for the invasion. The Laudabiliter began by stating that Pope Adrian IV, himself an Englishman, supported the idea of the English crown seizing control of Ireland. When Henry II landed in Waterford in 1171, he showed this document to local bishops and kings, greatly aiding his takeover.

However, Professor Duggan compared Henry's Laudabiliter to a papal decree given to Louis VII of France shortly afterwards, at a time when the French monarchy was planning an assault on Spain. By comparing the two documents, Duggan discovered that three key paragraphs were missing.

"The document given to King Louis begins by saying that the invasion of Spain is a good idea, " says Duggan, "but then there are three paragraphs when it says that France would need the approval of local kings and princes and that the pope could not give his approval unless this had been secured."

Duggan claims that the three key paragraphs were removed at the request of Henry prior to his departure from England.

"The entire basis of the controversy between Ireland and England for 800 years is based on a false document, " she concludes.

The editing was allegedly done by Gerald of Wales, whose family, the Fitzgeralds, had already seized much land in Ireland. Duggan claims that Gerald falsified the documents in the hope that the monarchy would grant his family further lands.

The programme, aired on BBC Radio 4 last Saturday, draws parallels between Henry's dodgy dossier and that of Tony Blair, who was accused of 'sexing-up' intelligence documents in a bid to legalise the British and US invasion of Iraq.

Papal decrees such as the Laudabiliter were the equivalent of UN resolutions in the 12th century, and had the ability to legalise the taking over of another country.

Charles Mosley, editor of Debrett's magazine, told the BBC that Duggan's research has the effect of justifying all Irish rebellion against British rule, even up to this day.

"The potential consequence of this lack of legality would be to justify the successive risings of the Irish against this lordship and the subsequent generations of Irish nationalists to continue that struggle, " he said.

DUP leader Ian Paisley told the programme that it was ironic that the English invasion of Ireland was the fault of the pope who, he said, could have acted to save Ireland.

"Irish people should not be sore at Ulstermen because it was the pope who handed them over to English rule, " said Paisley.

"The Roman Catholic church has never started a campaign to apologise to the Irish people for sending an English king in with all the authority of the pope, who was an Englishman, to do them down."

Sinn Fein refused to comment on the potential implications the revelations would have on the peace process.