Campaigners are to attempting to launch a legal challenge to the Christian coronation of future monarchs in an attack on the most powerful symbol of the union between church and state.

The National Secular Society (NSS) has instructed lawyers to investigate challenging the ancient religious rite under human rights legislation.

Keith Porteous Wood, the NSS executive director, said: "The country has changed out of all recognition since the last coronation and we should now be devising an investiture ceremony for the next head of state everyone can feel part of.

"A non-religious ceremony allows everybody to feel equally valued, and there is no reason that it should lack pomp or colourful ceremony simply because it is not religious, as with the ceremonies that take place in France or the US. "It is no longer appropriate to install the head of state in a religious service of one Christian denomination which – on a normal Sunday – less than 2% of the population attend."

Lawyers at Leigh Day, a firm of solicitors specialising in human rights, and Matrix chambers, have been instructed, he said. The NSS expects the challenge to focus on article 9 of the European convention on human rights, on the grounds that allowing the Church of England to perform such a ceremony limits the rights of conscience of the many people who do not subscribe to its beliefs, and article 14, which prohibits discrimination of any kind.

Arun Arora, the director of communications at the Church of England, said: "This flawed publicity stunt is mired in misunderstanding and confusion. The invocation of human rights legislation risks undermining the perception of vital laws purely for partisan purposes.

"To politicise the coronation in this way is a misguided and misjudged act by a campaign group of less than 10,000 members."

The coronation service conferred no powers, as the new monarch becomes monarch when their predecessor dies, and there is no law requiring a coronation, the NSS claimed. The Coronation Oath Act of 1688 stipulated elements of the ceremony, but not that it was required to take place, the campaign group believes. "These oaths suggest that the monarch is the person in the country with least freedom of religion– not of course that the Queen is complaining about this, and we doubt if [Prince] Charles would," said Porteous Wood.

He added: "We object almost as much to Prince Charles's intention to be 'defender of faith'; that is like saying he doesn't care about half of the population who are not religious or are religiously unconcerned.

"Britain is the last significant country to have a religious coronation. Why are we so much more hidebound in tradition than other countries?

"However, religious our head of state are, the ceremony is not just for them. It needs to be borne in mind that the country is becoming less and less Christian and religious, especially among the young."

An unnamed Church of England source was quoted in the Sunday Times as saying if the courts forbade the central element of the coronation, it would "not amount to disestablishment of the Church of England per se", but it would deal a "major blow" to the Christian religion. The source reportedly added it would also "hasten the removal of many of the remaining vestiges of church involvement in public life, such as prayers in parliament and at council meetings".

The NSS was involved in the successful campaign to give royals sons and daughters equal rights to succession. Last year, the high court ruled that the saying of prayers as part of a formal meetings of a council was unlawful following a judicial review initiated by the NSS.