Prince Charles should be called defender of the Christian religion, not all faiths, at his coronation service, an evangelical Christian group says.
Prince Charles wants to be "Defender of Faith" when he is king
The prince has said he wishes to become "Defender of Faith", not "Defender of the Faith", when he is king.
But the Evangelical Alliance says the ceremony for crowning him king is a traditional religious ceremony, and should be left or changed completely.
Clarence House said it would not be commenting on the suggestions.
'Duty to defend us all'
A modernised coronation service is said to have been planned which would incorporate other religious traditions.
But David Muir, policy director of the group - an umbrella organisation of evangelical Christians in the UK - said the coronation service was a religious ceremony, where the future sovereign swore to uphold the Protestant Christian faith.
"There needs to be a separate service where people of other faiths would also be included," he told BBC Radio Five Live.
"The actual service itself, until it is changed, is a religious service."
He added: "If they change that, that's fine."
The monarch already had a duty to "defend all our freedoms, all our rights", he said.
"Defender of the Faith" is a title used by all British monarchs since Henry VIII in 1521.
Prince Charles first expressed his wish to become Defender of Faith instead, in an interview in 1994.
He suggested the existing title implied the sovereign would protect only Christians.
The prince's position was seen by many as a reflection of multicultural Britain.
Earlier this year the Bishop of Rochester, Dr Michael Nazir-Ali, said all faiths could not be defended because of serious differences between them.
The suggestion is among about 100 recommendations made in a report from the Evangelical Alliance entitled Faith and Nation, to be published on Monday.
It covers areas including religious liberty, constitutional matters and the environment.
The Evangelical Alliance said Christians and other faiths gave their views for the report.