British Deputy Prime Minister Nick Clegg pledged Tuesday that the law on Royal succession will be changed at the "earliest opportunity" and that "whether the baby is a boy or a girl, they will have an equal claim to the throne."

Clegg announced that the U.K. and the other 15 Commonwealth nations have now formally agreed to change their laws.

'We know that the wishes of politicians are written in water.'—Royal historian Robert Lacey

Clegg also said that a Succession to the Crown Bill would be introduced in Britain's House of Commons as soon as the parliamentary schedule permits.

"Notwithstanding a few parliamentary turns of the wheel, this is now going to happen," Clegg said, adding that "the old-fashioned rules … have been swept aside."

"Born to Rule, Be It a Boy or a Girl" proclaimed the Daily Mail, which noted that the baby had "already made royal and constitutional history," even before it is born.

Not so fast, caution others.

A royal saga needs a touch of uncertainty, and experts point out that despite politicians' promises, the law giving males primacy in succession has not yet been changed — and the clock is ticking.

"We know that the wishes of politicians are written in water," said royal historian Robert Lacey. "Law only becomes law when the law is made — and the law has not been made."

The former Kate Middleton, who is less than 12 weeks pregnant, spent a second day in a London hospital Tuesday being treated for acute morning sickness. Photographers and camera crews from around the world camped outside, eager for news on the royal pregnancy.

Congratulations poured in from around the world at a piece of good news to follow Kate and William's lavish royal wedding in 2011 and Queen Elizabeth's Diamond Jubilee celebrations this year.

Officials say Kate and William's baby will displace Prince Harry, William's sibling, as third in line to the throne — and the child will stay there, even if she is a princess who later acquires a younger brother.

For centuries, preference was given to male heirs, so a first-born princess would be leapfrogged in the succession by a younger brother. As a result, there have been some 40 kings of England since the Norman Conquest in 1066, but only seven queens.

Key constitutional documents

Last year, leaders of Britain and the 15 former colonies that have the queen as their head of state informally agreed to establish new rules giving female children equal status with males in the order of succession — something that will require legal changes in each country.

"Put simply, if the Duke and Duchess of Cambridge were to have a little girl, that girl would one day be our queen," Prime Minister David Cameron said at the time.

Months passed with little progress. But the prospect of a royal birth next year seems to have focused political minds, at least somewhat.

In Britain, implementing the new rule means changing bits of several key constitutional documents, including the Bill of Rights and Coronation Oath Act of 1688, the 1701 Act of Settlement and the 1706 Act of Union with Scotland.

Lacey said that if it is not done by the time the baby is born, uncertainty is bound to remain. What if Kate has twins? Experts say the firstborn will be heir — but even that could be open to question.

"Say they have twins and a girl comes out first and 20 years later the boy turns out to be the more attractive character," Lacey said. "People will say that at the time the law meant the boy should have inherited."

Roman Catholics forbidden

Rebecca Probert, a professor at the University of Warwick's school of law, said there are other issues that the law should address to bring the monarchy up to date.

British monarchs are banned from marrying Roman Catholics, but not members of other faiths — something Clegg said the new law would change.

An heir also cannot marry without the monarch's permission, and can't marry in a civil ceremony — even though Prince Charles did just that when he wed his second wife, Camilla Parker Bowles, in 2005.

Probert said it's important "to have a clause in there confirming that monarchs are able to marry in the same ways that are open to their subjects."

But she thinks that may prove too complicated for lawmakers in the short term.

"They might decide in the interests of time to stick to the single issue of gender and leave the rest to a more convenient time — which tends to be never," she said.