LONDON: Prince William and his wife Catherine's baby will not be born until the summer, but changes to the rules of succession that would fundamentally affect its life are causing disquiet in the British establishment.

Prince Charles has led the warnings this week, echoed by senior clergy and peers, that the forthcoming legislation could have far-reaching consequences for the monarchy and the Church of England.

Ministers are pushing through a law to end male precedence in the line of succession, ensuring that if William and Catherine's first child is a girl, she would become queen even if she has a younger brother.

The change, which has strong public support, was agreed by Commonwealth leaders in October 2011, well before Catherine's pregnancy was revealed when she was forced to go to hospital in December last year with severe morning sickness.

With it, the monarchy moves into the 21st century, a direction reinforced by the palace's announcement on Wednesday that all William and Catherine's children would be titled Prince or Princess, not just their first-born son.

But now the law has reached the parliament, with a debate and vote due in the House of Commons on January 22, many in the establishment have openly expressed their concerns.

The law will also remove the ban on royal heirs marrying Roman Catholics, a specific provision that dates back to the split in Christianity, there is no such prohibition on heirs marrying into Islam, Hinduism or other faiths.

This presents potential problems when allied with the continuing ban on Catholics taking the throne, which is designed to protect the monarch's position as head of the Church of England, the so-called 'defender of the faith'.

It raises the possibility that an heir and their Catholic spouse would bring up Catholic children who would then be in line to inherit the throne, but be barred from taking it.

Friends of Charles, the heir to the throne and William's father, told a newspaper this week that he feared the government had overlooked the “unintended consequences” of the new law.

He is reportedly concerned about how the change would affect the delicate relationship between church and state, and how the rest of the aristocracy would be affected by ending male primogeniture.

Prime Minister David Cameron has denied there was any problem, telling lawmakers on Wednesday that the issue had been “settled and agreed” during “thorough contact” with Buckingham Palace.

But Prince Charles's remarks are not the only voice of caution.

A former archbishop of Canterbury, Lord George Carey, told another newspaper: “We must not have any ill-thought-through proposals because of the potential to upset a delicate constitutional balance.” Members of the House of Lords grilled deputy Prime Minister Nick Clegg on the reform at a hearing this week, questioning in particular why it was being pushed through parliament with only a day's debate by MPs.

Lord Peter Goldsmith, former attorney general under Tony Blair's Labour government, warned that changing centuries-old laws of succession was a “momentous decision” and fast-tracking it through was “worrying”.

One of his Conservative party colleagues, Lord Ian Lang, added: “The relationship between the church and the monarch is a very long-lived one which goes back far beyond the legislation you are amending.”

He warned: “If the question of the (monarch's role as) defender of the faith is brought into issue it then leads to the issue of whether the Church of England has to be disestablished.”

However, Clegg said the legislation had been worded very precisely “without in any way altering -- and we are completely confident of this, the status of the established church in this country, and the monarch as head of that church”.

The bill should pass easily through parliament as it is supported by all the major political parties, and will be followed by similar laws passed in the Commonwealth countries.

It will be retrospective, affecting all heirs born following the original agreement in October 2011.