Nobel peace prize winner Lord Trimble has accused the Irish government of using the Brexit negotiations to undermine the Good Friday Agreement.
The former first minister of Northern Ireland, who won the Nobel prize for his role in the peace process, accused Leo Varadkar's government of "riding roughshod" over the 1998 agreement.
He claimed that the Brexit process could result in Northern Ireland ending up as part of an "effective EU protectorate".
The former Ulster Unionist Party leader, now a Conservative peer, said in a foreword to a think tank report that the achievements of the Good Friday Agreement were being put at risk.
In the report, produced by the centre-right Policy Exchange, Lord Trimble said: "It is clear to me that the Irish side in the Brexit negotiations is undermining the 1998 Good Friday Agreement, riding roughshod over its terms and violating its spirit. "
The report said the European Union and Irish demands "go well beyond the avoidance of a hard border".
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Brexit talks have reached an impasse over the EU's "backstop" plan which would see Northern Ireland effectively remaining in the customs union and single market unless alternative arrangements were found to prevent a hard border.
Lord Trimble said: "There is a genuine risk that Northern Ireland will end up as part of an effective EU protectorate, without the say-so of the Northern Ireland Assembly.
"This would be an appalling breach of the principle of consent, which runs through the (Good Friday) agreement."
The Policy Exchange report suggested that "one possible way out of the current impasse is for both sides to sign a declaration that future trade talks ... will include an 'Ireland chapter' in which both sides undertake to use their best efforts to ensure an invisible border" and preserve existing measures of co-operation.
The report was written by Graham Gudgin, who was an aide to Lord Trimble as first minister, and Ray Bassett, who was on the Irish talks team during the Good Friday Agreement negotiations.
They said: "Following the principles of the Good Friday Agreement should be at the heart of both UK and EU approaches to agreeing future co-operation over the Irish border.
"It is currently Theresa May's stance which is most closely aligned to those principles - while the EU has fundamentally misinterpreted the principle of consent which underpins them.
"The current impasse could be resolved if both sides agree that in future trade talks both sides undertake to use their best efforts to ensure an invisible border and to preserve all existing measures of cross-border cooperation agreed under the auspices of the Good Friday Agreement."