Gordon Brown, David Cameron and Nick Clegg don’t all agree on much. But they’ve all spoken out against the antiquated laws which prevent Roman Catholics and women having equal access to the throne.
Under current rules, if Prince William and Princess Catherine first born child were female, she would not have the same right to the throne as a younger brother. But on Wednesday the prime minister said he had written to 15 Commonwealth countries to change the outdated legislation.
Huff Post UK talked through what’s being proposed with Bob Morris, an honourary senior research associate at UCL’s constitution unit.
So what’s changing - and why is it so diplomatically complicated?
The government want to ensure “gender equality” by allowing Prince William’s first child, whatever its gender, to be entitled to the throne. They also want to change the rule which prevents those who marry Roman Catholics taking the crown.
The prime minister said: "This rule is a historical anomaly - it does not, for example, bar those who marry spouses of other faiths - and we do not think it can continue to be justified".
They also want to change an old rule which requires any descendants of King George II to get the monarch’s permission before marrying.
But even that won’t mean non-protestants can take the throne, Bob Morris explains: “There is a third requirement that they're not erasing, which requires them to be in holy communion with the Church of England. To remove the explicit requirement about Roman Catholics doesn't mean they will be able to succeed the throne. It rules out Quakers, and people of other faiths.
“They are very reluctant to tamper with the current form of establishment of the Church of England.”
Why has this not been changed before?
David Cameron is not the first leader who has tried to change the rule of sucession: In 2008 Gordon Brown's government said it was looking at the "antiquated" rules surrounding male primogeniture and which prevent Catholics from taking the throne. It's a slow process, mostly because it depends on 15 other nations agreeing to a change in the laws.
And then there’s the unintended consequences. Morris explains: “There is a lot of reluctance in having to unwind all of this, on the part of governments.
“There's probably still a fair degree of anti Roman Catholic feeling still in the country, there's a reluctance to feel you might have a head of state who owes allegiance to the foreign papal state. Why stir up something when you've got so much on your plate?”
So who has to say yes?
Many Brits won't have heard of Tuvalu, a small island in the Pacific Ocean with a population of just over 10,000. But it's one of the 15 nations that has to sign off on any changes to the royal succession.
"The problem is the Queen is head of state of 16 countries. if you wish to change the rules of succession then you'll want to do it in a uniform way otherwise you can have different people being different heads of state.
“The problem for some of the realms is they are independent states and will have constitutions of their own”, says Morris.
So what could go wrong?
The states could refuse, although Morris says there would have been a significant degree of behind-the-scenes diplomacy before David Cameron sent the letter.
It’s also difficult for other countries to change their constitutions: “It's possible that in Australia the central government can lead things but there would have to be legislation in each of the states.
“But how long it would take is less important as to whether they would agree to do it. in the UK, we can change our constitution simply by statute. Whereas in other countries there are differences”, Morris explains.
In the UK we would have to amend three acts: the Bill of Rights and Coronation Oath Act of 1688, the 1701 Act of Settlement and the 1706 Act of Union with Scotland.
Then there’s anti-monarchy feelings: “It will reignite concerns in Australia about moving to a Republic although [prime minister] Julia Gillard has said they are behind the changes.
“There may be some attempts to make political capital out of all this. I imagine there has been a lot of diplomacy going on behind the scenes before Cameron wrote his letter.
“Will all of this emulate more republicanism? In a sense the UK is a republic, it just happens to have a hereditary head of state.”
Morris adds: “There may be some feeling amongst some of the Commonwealth countries that there should be a question about who the head of the Commonwealth should be that might change with Charles.”