The current line of succession to the throne is Prince Charles, followed by his sons Prince William and Prince Harry.

The U.K. Parliament regulates the line of succession to the British throne, using guidelines laid out in the Bill of Rights (1689) and the Act of Settlement (1701).

In October 2011, Commonwealth leaders unanimously agreed to remove the centuries-old gender discrimination rule that favours first-born sons over older daughters in the order of succession to the throne.

Prince William, second in line to the throne, and his wife, Kate, the Duchess of Cambridge, announced Dec. 3 that they are expecting their first child in 2013. The gender of the child is not known.

While the new law has not yet been introduced in the British Parliament, it's already de facto law, says a spokesperson for the British Cabinet Office.

If William and Kate's first child is a girl, she will be able to succeed the throne, no matter if she eventually has a younger brother.

"Even though the bill hasn't actually been introduced here yet that doesn't matter. It's effectively law already," the cabinet office spokesperson said.

The cabinet office hopes to introduce a bill very soon, the spokesperson said.

The move came after British Prime Minister David Cameron sent a letter to Commonwealth leaders — including Prime Minister Stephen Harper — proposing to change British law so that a male heir no longer takes precedence over a female heir.

The Commonwealth leaders also agreed to lift a ban on the monarch being married to a Roman Catholic.

According to the Act of Settlement, only Protestant members of the Royal Family who are descendants of Princess Sophia (1630-1714), the Electress of Hanover, a granddaughter of James I, can be considered for the throne.

The sovereign is required to take an oath to defend the Church of England and the Church of Scotland. A Roman Catholic is disqualified from succession to the throne.

There are more than 900 people in line for the British throne; the first 510 on the list are descended from Queen Victoria (1819-1901).

Rules specifying who inherits the throne, now based on the 1701 Act of Settlement, are not easy to change — particularly because they involve all 16 Commonwealth countries where Queen Elizabeth is head of state.

Harper has said the "obvious modernizations" were long overdue. He hasn't given a timeline for when legislation will come before Parliament for approval, but said he hopes it will be adopted quickly.

Line of succession
Sovereign: Elizabeth II (b.1926)
1. The Prince of Wales (b. 1948), Queen Elizabeth II's eldest son (Prince Charles)
2. Prince William of Wales (b. 1982), the Prince of Wales's elder son
3. Prince Henry of Wales (b. 1984), the Prince of Wales's younger son
4. The Duke of York (b. 1960), Queen Elizabeth II's second son (Prince Andrew)
5. Princess Beatrice of York (b. 1988), the Duke of York's elder daughter
6. Princess Eugenie of York (b. 1990), the Duke of York's younger daughter
7. The Earl of Wessex (b. 1964), Queen Elizabeth II's youngest son (Prince Edward)
8. Viscount Severn (b. 2007), the Earl of Wessex's son (James)
9. Lady Louise Windsor (b. 2003), the Earl of Wessex's daughter
10. The Princess Royal (b. 1950), Queen Elizabeth II's daughter (Princess Anne)
11. Peter Phillips (b. 1977), the Princess Royal's son
12. Savannah Phillips (b. 2010), Peter Phillips' daughter
13. Isla Phillips (b. 2012), Peter Phillips' daughter
14. Zara Phillips (b. 1981), the Princess Royal's daughter
15. Viscount Linley (b. 1961), grandson of King George VI through his daughter Princess Margaret
16. Charles Armstrong-Jones (b. 1999), Viscount Linley's son
17. Margarita Armstrong-Jones (b. 2002), Viscount Linley's daughter
18. Lady Sarah Chatto (b. 1964), granddaughter of King George VI through his daughter Princess Margaret
19. Samuel Chatto (b. 1996), son of Lady Sarah Chatto
20. Arthur Chatto (b. 1999), son of Lady Sarah Chatto
21. The Duke of Gloucester (b. 1944), grandson of King George V through his son Prince Henry, Duke of Gloucester
22. Earl of Ulster (b. 1974), son of the Duke of Gloucester
23. Lord Culloden (b. 2007), son of Earl of Ulster
24. Lady Cosima Windsor (b. 2010), daughter of Earl of Ulster
25. Lady Davina Lewis (b. 1977), elder daughter of the Duke of Gloucester
26. Senna Lewis (b. 2010), daughter of Lady Davina Windsor
27. Lady Rose Gilman (b. 1980), younger daughter of the Duke of Gloucester
28. Lyla Gilman (b. 2010), daughter of Lady Rose Gilman
29. The Duke of Kent (b. 1935), grandson of King George V through his son Prince George, Duke of Kent
30. Lady Amelia Windsor (b. 1995), daughter of George, Earl of St. Andrews
31. Lady Helen Taylor (b. 1964), daughter of the Duke of Kent
32. Columbus Taylor (b. 1994), son of Lady Helen Taylor
33. Cassius Taylor (b. 1996), son of Lady Helen Taylor
34. Eloise Taylor (b. 2003), daughter of Lady Helen Taylor
35. Estella Taylor (b. 2004), daughter of Lady Helen Taylor
36. Albert Windsor (b. 2007), son of Lord Nicholas Windsor
37. Leopold Windsor (b. 2009), son of Lord Nicholas Windsor
38. Lord Frederick Windsor (b. 1979), son of Prince Michael of Kent
39. Lady Gabriella Windsor (b. 1981), daughter of Prince Michael of Kent
40. Princess Alexandra, Lady Ogilvy (b. 1936), granddaughter of King George V through Prince George, Duke of Kent