He said it was ''wholly unacceptable'' for Tehran to seek a nuclear weapons capability and insisted there could be ''no alternative'' to military force ''if they continue to develop nuclear weapons''.
Speaking to BBC journalist Andrew Marr, Mr Blair said: ''I am saying that I think it is wholly unacceptable for Iran to have a nuclear weapons capability and I think we have got to be prepared to confront them, if necessary militarily.
''I think there is no alternative to that if they continue to develop nuclear weapons. They need to get that message loud and clear.''
In the interview, to be shown on BBC2 on Wednesday night, Mr Blair also said he was ''desperately sorry'' for the relatives of those killed in Iraq but insisted leaving Saddam Hussein in power would have been a ''bigger risk'' to security than removing him.
He was ''sorry for the lives cut short'' but did not regret the decision to go to war.