Bizarre reason why Meghan Markle and Prince Harry won't have full custody of their children

The newlyweds were overjoyed to announce they are expecting a royal bub while on their travels to Australia - but it seems they won't ever have full custody of the child.

Much like the other royal children, Queen Elizabeth II will have legal custody of Harry and Meghan's unborn baby.

It is an age-old arrangement which dates back to the 1700s, ensuring the sovereign has legal custody of the minor grandchildren. 

Royal expert Marlene Koenig told news.com.au the rule was passed when King George I wanted to be the legal guardian of his grandchildren.

'This goes back to King George I [who ruled in the early 1700s], and the law’s never been changed. He did it because he had a very poor relationship with his son, the future King George II, so they had this law passed that meant the King was the guardian of his grandchildren,' Ms Koenig said.

The law still stands today and means the Queen has the final say on parenting decisions.

It also means the royals need express permission from Her Majesty to take the children on overseas trips.

When Princess Diana wanted to take Harry and William on a trip to Australia shortly before her death, the Queen did not give her approval.

Prince Charles also needed permission to send a teenage Prince William to a camp in the US in the late 1990s.

In the event of the Queen's death, legal custody will be passed over to the children's grandfather, Prince Charles.