The government should reconsider its decision to allow sixth-formers to opt out of religious worship, the Catholic Education Service (CES) says.
Schools must provide a daily act of collective worship
The Education and Inspections Bill has been amended to let children over 16 withdraw from prayers, rather than their parents having to request it.
But CES chief executive Oona Stannard said this change should be reversed for state-funded faith schools.
Prayers were the "part of the package" and "integral to the ethos", she added.
Under current laws, state schools in England and Wales "must provide daily collective worship for all registered pupils" that is "wholly or mainly of a broadly Christian character".
Parents can choose to withdraw their children from this.
In July, the Lords agreed the Education and Inspections Bill - which is still going through Parliament - should be amended to allow those aged 16 and over to opt out.
Ms Stannard said: "If a student chooses a Catholic school or college for their post-GCSE studies, it is important that they recognise that the prayer life of the school is an essential part of the package.
"Collective worship and RE lessons are integral to the ethos and success of every Catholic school and college; without them you would be talking about a very different place - a community in which students and parents would be less inclined to take part."
Ms King said: "Coercion is not what we're about, but collective worship and RE lessons have a very important role to play in the life of every Catholic school and sixth form college."
Keith Porteous Wood of the National Secular Society said: "The Catholic Education Service appears to think that religious schools should be exempt from the duty to apply human rights in its schools.
"But these are publicly-funded institutions and human rights are universal. There can be no exemptions, and especially not for religious bodies, who, history has shown, are not above abusing human rights."
In the Lords last month, Liberal Democrat Baroness Walmsley argued that it was unfair that 16-year-olds could pay tax and get married, yet could not refuse to worship.
The government's decision to allow the opt-out followed a protest by a group of pupils at St Luke's College in Bexley, south London - a Catholic sixth-form college - over compulsory attendance at Mass.