The laws of succession, which place Prince William immediately in line to inherit the British throne after his father Prince Charles, are riddled with curious rules designed in troubled times.

The eldest son of Queen Elizabeth II's eldest son, William, who marries his university sweetheart Catherine Middleton on April 29, has been second in line since birth.

So far, so clear. But the next generation could force a radical overhaul to the succession rules.

The laws on who can accede to the throne are straightforward but controversial due to the male primogeniture rule -- and the exclusion of Catholics.

Any children from their marriage will come immediately behind William, with the eldest boy, or the eldest daughter if there are no sons, coming directly in line to inherit the throne.

If William's first-born is a girl, the laws will likely be altered. However, the change has not happened so far because tinkering with the rules is highly problematic, requiring legal agreement across the parliaments of all 16 Commonwealth realms.

The concern is that once the subject is opened up, republicans -- particularly strong in Australia -- could hijack the process and try to oust the monarchy altogether.

Now a more pressing issue, British Prime Minister David Cameron and his New Zealand counterpart John Key said at the weekend they were in favour of changing the rules, but neither seemed optimistic of it happening soon.

"Everyone in the front line of politics agrees that this does need to change," Cameron said, "but it clearly does take some time".

A spokeswoman for Britain's Cabinet Office told AFP that discussions between the realms were under way but it was "a complex and difficult matter that requires careful and thoughtful consideration".

The immediate line of succession includes Queen Elizabeth's four children and their offspring:

1. PRINCE CHARLES, THE PRINCE OF WALES

-- Born 1948. Eldest son of Queen Elizabeth.

2. PRINCE WILLIAM OF WALES

-- Born 1982. Charles' eldest son.

3. PRINCE HARRY OF WALES

-- Born 1984. Charles' second son.

4. PRINCE ANDREW, DUKE OF YORK

-- Born 1960. Queen Elizabeth's second son.

5. PRINCESS BEATRICE OF YORK

-- Born 1988. Andrew's eldest daughter.

6. PRINCESS EUGENIE OF YORK

-- Born 1990. Andrew's second daughter.

7. PRINCE EDWARD, EARL OF WESSEX

-- Born 1964. Queen Elizabeth's third son.

8. JAMES, VISCOUNT SEVERN

-- Born 2007. Edward's son.

9. LADY LOUISE WINDSOR

-- Born 2003. Edward's daughter.

10. PRINCESS ANNE, THE PRINCESS ROYAL

-- Born 1950. Daughter of Queen Elizabeth.

11. PETER PHILLIPS

-- Born 1977. Anne's son.

12. SAVANNAH PHILLIPS

-- Born 2010. Phillips' daughter.

13. ZARA PHILLIPS

-- Born 1981. Anne's daughter.

Queen Elizabeth's descendants are followed by those of her late sister princess Margaret, her only sibling.

Drawn up in an era of religious strife, the Act of Settlement 1701 states that only Protestant descendants of Sophia of Hanover, the mother of king George I, can accede to the throne. Catholics and anyone marrying a Catholic are specifically excluded.

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