The Right Rev Michael Nazir-Ali, the Bishop of Rochester, said that differences between religions made it impossible to defend all of them.
The Prince first said that he wanted to be known as "Defender of Faith" - as opposed to "Defender of the Faith" - in 1994, when he suggested that the monarch's traditional title implied that he or she would protect only Christians.
Speaking on Radio 4's Today programme yesterday, Dr Nazir-Ali said that such a change would be impossible. "The coronation service is such that whoever takes the oaths actually takes oaths to defend the Christian faith," he said.
"If, by saying that, he meant that he wanted to uphold the freedom of people of every faith, then I have no quarrel with that. But you can't defend every faith, because there are very serious differences among them."
In an interview earlier this week, the bishop - who was born a Muslim - called on fellow Anglicans to reassert Britain's "Christian character" and resist the trend towards a "multi-faith mish-mash".