The birth of a royal baby boy is a cause of celebration for Britain and the Commonwealth but it may remove the sense of urgency around a historic change in the law on female succession, say academics.

Ending male primogeniture had been talked about for decades and Prince William and Kate's wedding in 2011 jolted governments into action to ensure that if their first-born was a girl, its gender could not keep it from the throne.

The baby was widely expected to be a girl, but now that the royal couple have produced a healthy male heir, the pressure to end legal wrangles in Australia and Canada and push through the new law has eased slightly.

"The fact that there is now no apparent urgency may affect the tempo of the change," Bob Morris, a research fellow at the Constitution Unit of University College London, told AFP.

"That is a process which is going on rather slowly."

The little boy will be third in line to the throne, after Queen Elizabeth II's eldest son Prince Charles and Charles's eldest son Prince William.

It is the first time since 1894 that three direct heirs to the throne have been alive at the same time.

"So although men have taken preference up until now, the prospect of three men in a row -- Prince Charles, Prince William, and now the new baby -- is actually rather unusual," said Clarissa Campbell Orr, historian of the monarchy at Anglia Ruskin University.

But Tina Brown, editor-in-chief of the Daily Beast website, said the famously polished Kate had done the "perfect thing" by giving birth to a royal boy.

"Kate can do no wrong! Now the royals can stop pretending they were fine with a girl 1st!," she wrote on Twitter.

The position is a far cry from the scenario that British Prime Minister David Cameron's government envisaged when it pushed through the equal succession laws.

The premiers of Britain, Australia, Canada, New Zealand and the 12 other countries where Queen Elizabeth reigns reached agreement in principle at the last Commonwealth summit in October 2011.

The law change must be unanimous and identical in each country to avoid ending up with different monarchs in different states. It can only come into effect when all the realms have ratified it.

By sheer coincidence, formal agreement to proceed with legislation in Britain was received on the same day last December that Kate's pregnancy was announced. Sped through parliament in Britain, the Succession to the Crown Act 2013 gained royal assent on April 25.

But the process is not proving as straightforward as had been hoped, with challenges thrown up by Quebec in Canada and Queensland in Australia.

If and when it is passed, though, the legislation will be backdated to apply from the 2011 Commonwealth summit agreement onwards.

"It's not a race against time," said a spokesman for Britain's Cabinet Office ministry. "All the realms agreed that they would do this."

It also removes the ban on a future monarch marrying a Roman Catholic, and limits the number of those in the line needing the monarch's permission to marry to the first six.

The baby pushes William's younger brother Prince Harry, 28, down to fourth in line to the throne. Harry, a helicopter pilot with the British army, will drop further down with any future children that William and Kate may have.

In a new move, the baby boy bears the title of prince or princess and will be called "royal highness" from birth, following letters patent issued by Queen Elizabeth on December 31 last year.

Just as there are no set duties for the heir to the throne, there are even fewer for those further back in the queue, with the royal baby facing a lengthy wait for the crown.

Patrick Jephson, former chief of staff to William's mother Diana, told AFP: "One of the big problems facing the future of the British royal family is this issue of a surplus of heirs.

"As long as most British people can remember, they have had the same sovereign. A lot of young people, I would imagine, are keener there should be a king and queen of their own generation."