The Government must demand legally binding guarantees from the United States that it is not illegally transporting prisoners through Shannon airport, the Council of Europe's top legal advisers have made clear.
The detailed legal advice was sought last December by Dick Marty, the Swiss head of the Council of Europe's investigation into allegations that the US has sent prisoners for torture in the Middle East and elsewhere following the Afghan and Iraq invasions.
The Venice Commission's legal opinion would weigh heavily with the European Court of Human Rights if a legal challenge was taken by anyone who claims that they were tortured after being flown through Shannon.
More than 50 flights chartered by the US Central Intelligence Agency have landed at Shannon over the last couple of years.
Following persistent complaints from the Opposition and lobby groups, the Government has received repeated guarantees from US figures, including US secretary of state Condoleezza Rice, that prisoners have not been transported through Shannon.
Up to now, the Government has insisted that the quality of the guarantees from a friendly nation cannot be impugned without causing major diplomatic offence.
However, the Venice Commission has said that Council of Europe states must inspect aircraft landing in their jurisdictions if there are "serious reasons" to believe that prisoners bound for torture elsewhere are on board.
The commission has raised the bar even further by warning European states that they must act to protect prisoners from torture even if they are being carried on board flights that cross their airspace but do not land.
European states would be in breach of the European Convention on Human Rights if they did not act to ensure the release of the prisoner, even if that meant forcing the aircraft to land.
The Venice Commission says aircraft owned or chartered by states are treated differently under the Chicago Convention, which governs international aviation, to those operating as civilian aircraft.
The Irish authorities are entitled to search civilian aircraft "without unreasonable delay", though official aircraft can only be inspected with the agreement of the aircraft's captain, bar exceptional circumstances.
European states must impose new conditions on US flights landing in their territories or using their airspace if they have concerns, as a precondition for issuing diplomatic clearances, the commission says.
Civilian aircraft chartered by the Central Intelligence Agency that have used Shannon since 2001 could have been searched without any restrictions unless the US had declared in advance to the Irish authorities that the aircraft were on official duties.