News of the split comes as it has emerged that the threat of attack to US planes at Shannon Airport has forced the Irish government to spend almost €10m of taxpayers' money to protect them.

Green councillor Brian Meaney, who had a parliamentary question asked on his behalf to reveal the costs, this weekend said that the spend is largely unnecessary. He launched a stinging attack on his own party colleagues in government for failing to do enough to ensure rendition is not taking place at Shannon.

Speaking to the Sunday Independent, Brian Meaney said: "No civilised society can defend the practice of rendition. Nobody should be subjected to that treatment. The Programme for Government ensured that Ireland would not tolerate the practice. Nothing has been done and bottom line is that all suspected flights should and must be searched."

But it appears that his senior colleagues are actually in agreement with him. A senior Green Party spokesman told Fifth Column: "Cllr Brian Meaney is an excellent local representative and we share his concerns. The issue was dealt with in the Programme for Government. The matter is the subject of ongoing contact between the Green ministers and the Minister for Justice."

According to latest figures, 262,816 troops passed through the airport in 2007, of which 171,000 travelled between July and December. Since 2005, Shannon has made an estimated €23.5m from allowing the US flights to use the airport as a transfer destination and refuelling stopover.

Now, according to latest figures obtained, the State has been forced to spend millions on protecting the planes, which some believe are involved in the process of rendition, as a result of repeated breaches in the airport perimeter fence and attacks on a number of planes.

The total cost, including salaries, of policing arrangements at Shannon Airport for 2006 was €4,695,737 and for January to November 2007 was €3,134,011. Given the ongoing transit of troops and the suspected use of Shannon as a rendition route, the security costs will top the €10m mark before the end of the summer and could be as high as €12m by the end of the year.

According to the Department of Justice, the costs include policing the perimeter of the airport.

Defending the costs, Justice Minister Brian Lenihan said: "Gardai are statutorily required to provide policing services for the State with the aim of preserving public order and protecting life and property. Resources are allocated by the Commissioner according to assessed threat, so it is the latter which ultimately determines the associated costs. I remain satisfied that Garda policing arrangements at Shannon Airport continue to be both necessary and appropriate."

Daniel McConnell