The Taoiseach, Mr Ahern, risks prosecution in the International Criminal Court if he does not ban US military planes from refuelling at Shannon Airport, an Irish peace activist has warned.
Mr Ahern could be held accountable for "aiding and abetting" crimes against humanity under Article 7 of the Treaty of Rome if he assists "any party engaged in this illegal war", Mr Ruadhan Mac Eoin has said.
"It is now impossible for the impending US war in Iraq to have the sanction of the United Nations, and so any such war will now inevitably be in violation of the international legal order.
"Hence any deaths that occur will not just be wrong and immoral but also quite clearly illegal."
Mr Mac Eoin, a peace activist and spokesman for the Carrickmines Castle/M50 protesters, said he has delivered a letter to the Taoiseach's office calling on Mr Ahern to confirm that he will "give no assistance of any kind" to the war effort.
If Mr Ahern does not give such an assurance, Mr Mac Eoin will request that the International Criminal Court begin an investigation against the Taoiseach and the Government for "aiding and aiding and abetting crimes against humanity", the letter said.
A spokeswoman for the Government was last night unable to confirm if Mr Ahern had received Mr Mac Eoin's letter.