Allawi, interviewed in London, said he did not expect the accord to lead to a stable government because it would not be the power-sharing administration it had been portrayed as.
"The formula for power sharing has been distorted and the issue of devolution has been distorted so I am not sure whether a coherent government (can be formed)," he told Reuters.
"Still we have some time to discuss issues and to see if this will happen or not," he added. Asked if the government could last long, he replied "No".
He said he would not join a new cabinet. "I am not going to be part, in any case, of the council of ministers. It is not currently suggested or offered or the Iraqiya list want me to be a member".
He said he had not decided whether to accept the new senior role offered to him.
Allawi played down the walkout from parliament by members of his bloc last week, saying the disagreements behind it had been settled.
Allawi, who said he was in London for his daughter's wedding, added that he would not return to Baghdad for the opening of parliament on Nov. 21.
Iraq needs a stable government to rebuild infrastructure and exploit its vast oil wealth while violence ebbs, seven years after the U.S.-led invasion that ousted dictator Saddam Hussein.
Under the power-sharing deal, politicians divided the three top posts -- prime minister, president and speaker of parliament -- among the main ethnic and sectarian political blocs.