Thrilled Britons bet on baby names as congratulations poured in on Tuesday after the Duke and Duchess of Sussex welcomed the royal family's first known mixed-race member.

Prince Harry on Monday announced the birth of the couple's first – a son who will be seventh in the long line to the British throne.

"This little thing is absolutely to die for," Harry, 34, said with a giggle, delighted but somewhat overwhelmed.

Harry "was so genuine," remarked Mark Schultz, 57, an American queuing outside Windsor Castle with his wife Jeanelle and other tourists on Tuesday.

"Any time there's a royal baby it's exciting, but Meghan, she's one of ours," Jeanelle added.

BABY SUSSEX

Britain has a long and proud tradition of betting on just about everything, and the new arrival had the bookies busy for months.

Some shops stopped taking punts on female names because everyone – somewhat inexplicably, based on rumours and anonymous tips – assumed that it would be a girl.

The odds changed radically Monday, with Albert, Oliver, Edward, Henry and Charles all surging to the top.

Arthur – a traditional royal name that is among the middle names of the baby's grandfather, the Prince of Wales, and his uncle the Duke of Cambridge – is the favourite with many of the bookies, while Alexander and James are also popular.

UK royal names are normally chosen from a relatively restricted pool of those used by past kings and queens.

But Meghan and Harry, the Duke and Duchess of Sussex, are anything but normal royals.

"Meghan Markle and Prince Harry to choose UNUSUAL name for baby that UNIFIES the US and UK," The Daily Express wrote, sparking a wild guessing game on Twitter filled with puns and jokes.

For now, British television stations refer to the new arrival as simply "Baby Sussex".

ROYAL BIRTH

The Palace said the Queen, the Duke of Edinburgh and the Prince of Wales were all "delighted with the news".

The baby, who has not been named yet, is seventh in line to the throne, behind the Prince of Wales, the Duke of Cambridge and his children - Prince George, Princess Charlotte and Prince Louis - and Prince Harry.

He is the Queen's eighth great-grandchild.

The baby is a dual citizen, having automatically acquired US citizenship, via Meghan, at birth.

Speaking after the birth, Harry said they planned to present the baby to the media in two days' time "so everyone can see" him.

He said Meghan and the baby were doing "incredibly well", adding that they were still thinking about names for the infant.

Buckingham Palace said the duke was present for the birth.

CONGRATULATIONS

The entire world seemed to celebrate along with the happy couple, reflecting both the expanse of the former British empire and the public's fascination with how Meghan will get on with her new in-laws.

On the border between the United States and Canada, the Niagara Falls waterfall was lit up in blue.

Meghan's mother, Doria Ragland, was said to be "overjoyed", while former US First Lady Michelle Obama said she and Barack were "thrilled".

Prime Minister Theresa May, Labour leader Jeremy Corbyn and Scotland's First Minister Nicola Sturgeon were among the politicians to offer their congratulations to the royal couple on Twitter.

Archbishop of Canterbury Justin Welby, who married the couple in May last year, said: "May God bless the new family with love, health and happiness."

The choir that sang at Prince Harry and Meghan's wedding wrote a lullaby for the new baby.

Congratulations also came from celebrities and famous figures in the United States, where Meghan was born.

Harry and Meghan received congratulations from former US first lady Michelle Obama.

Meghan's estranged father Thomas Markle told The Sun: "All newborn babies bring entire families happiness and joy - they can even bring broken families back together again.”

Patrick Adams, who played Meghan's husband in the US TV series "Suits," sounded more jovial.

"Just heard that the world just got heavier by 7 pounds and 3 ounces," Adams tweeted.

"Much love to him and his incredible parents."

GODPARENTS

Even as the congratulatory messages poured in, newspapers debated on who would be named the little prince’s godparents.

The Mail claims it could be a 'Toffs v Celebs' race, speculating that the shortlist could contain A-listers such as George Clooney, Serena Williams and the US television host, Gayle King.

The Mirror believes between five and eight godparents could be chosen, with Princess Eugenie and Zara Tindall in the running.

And the Sun goes as far as offering odds on the matter, with Catherine, Duchess of Cambridge at 4-to-1, Bollywood star Priyanka Chopra a 25-to-1 shot, and Oprah Winfrey a 66-to-1 outsider.