The King should be officially separated from the Church of England, campaigners have said, after official figures this week showed the country is no longer majority-Christian.
Census results from the Office for National Statistics (ONS) revealed the number of people describing themselves as Christian in England and Wales fell below 50% for the first time, at just 27.5 million people or 46.2% of the population – down from 33.3 million or 59.3% of the population in 2011.
The data also showed the number of people who said they had "no religion" had increased markedly to 22.2 million, around one-third of the population.
Now anti-monarchists and non-religious groups have raised questions about King Charles's role as supreme governor of the Church of England.
Graham Smith, CEO of the Republic campaign group that wants to see the monarchy abolished and the King replaced with an elected head of state, said the data shows the current system is unsustainable.
"How can a head of state be said to be impartial and representing all of us when they are so closely linked to one religion, a religion that is increasingly a minority interest... Clearly the head of state being head of church is unsustainable," Smith told Yahoo News UK.
"It has never been right that one religion is given a privileged place in our constitution, but now more than ever that needs to end.
"Charles has a unique opportunity, in the run-up to the coronation, to agree to a separation of his role from the church, allowing a wider debate to be had about separation of church and state."
The 2021 survey, carried out on March 21, was filled out by more than 24 million households across England and Wales. It showed:
Increases in the proportion of people describing themselves as Muslim (up from 4.9% to 6.5%) and Hindu (from 1.5% to 1.7%)
In a third of households (32.7% – 8.1 million) all members reported the same religion, while 13.7% (3.4 million) have a mix of religious and non-religious people and in 1.1% (285,000) at least two different religions were reported
In a fifth of households (20.4% – 5.1 million) all members said they had no religion
London remains the most religiously diverse region of England, with just over a quarter (25.3%) of people on the day of the 2021 census reporting a religion other than Christian.
Stephen Evans, of the National Secular Society, said it was "absurd and unsustainable for the head of state to swear to uphold and promote a single religion which less than half the population follow".
The monarch has been the head of the Anglican Church – for the most part – since the break with Rome and Papal authority in 1534. Henry VIII reformed the church because the Pope would not accept an annulment of the king's marriage to Catherine of Aragon.
The monarch's official title is defender of the faith and supreme governor. However, he is not the head of the Church of Scotland, which is presbyterian and only recognises Christ as its head.
In 1994, Charles said he would be "defender of faith" rather than of "the faith". This was met with controversy.
Speaking to BBC Radio 2 in 2015, he expanded on the matter: "I mind about the inclusion of other people’s faiths and their freedom to worship in this country. And it's always seemed to me that, while at the same time being defender of the faith, you can also be protector of faiths."
Charles also said the Anglican Church "has a duty to protect the free practice of all faiths in this country".
For some campaigners, this isn't enough. Instead, they want to see a complete separation of the church from the state.
Humanists UK, which ran a campaign ahead of the two most recent censuses encouraging non-religious people to tick the form's "no religion" box, said the result should be a "wake-up call which prompts fresh reconsiderations of the role of religion in society".
CEO Andrew Copson said: "No state in Europe has such a religious set-up as we do in terms of law and public policy, while at the same time having such a non-religious population. Iran is the only other state in the world that has clerics voting in its legislature."
He added: "This census result should be a wake-up call which prompts fresh reconsiderations of the role of religion in society."
Watch: Proportion of population describing themselves as Christian falls below 50%