The newest member of Britains royal family, due to make an entrance later this month when Prince Williams wife Kate gives birth to the couples second baby, will be taking on one of its toughest roles – life as spare to the heir.

Some 21 months after the birth of Prince George to a global fanfare which made him one of the most famous babies in the world, the hugely popular royal couple are due to celebrate the arrival of a younger brother or sister in the next few weeks.

George, born 21 months ago is expected one day to be monarch following in the footsteps of Queen Elizabeth, his grandfather Prince Charles, and his father. But the younger offsprings future is less clear.

The spare role is an undefined one that allows more freedom than that accorded to a future king or queen. But it also attracts massive public interest and scrutiny, while the possibility remains of having to step into the shoes of the future heir should calamity befall the elder sibling.

I think the difficulty is just that one thing - that you never quite know if youre going to be called upon to become king, or instead have what you if youre being really cynical could say a relatively pointless life, royal historian Hugo Vickers told Reuters.

Both the queens grandfather George V, whose elder brother died aged 28, and her father George VI, whose elder brother Edward VIII abdicated to marry American divorcee Wallis Simpson, were spares who did become monarchs. For those for whom the top job never comes, life as a spare is not easy.

I think its incredibly difficult for the younger sibling to a future monarch because they are in the public eye but they dont have the protection perhaps and they dont have the advice and they dont have the role of king or queen, said Claudia Joseph, author of William and Kates Britain.

Princess Margaret, the queens younger sister who died in 2002, was a prime example. Renowned as a beauty in her youth who partied with a high society set, her private life generated intrigue for the media. She is best remembered for falling for dashing air force officer Peter Townsend when protocol dictated that a princess could not marry a divorced man.

Instead she later married photographer Anthony Armstrong-Jones, a controversial choice because of his non-aristocratic status. They divorced in 1978, in the first such split in the inner royal circle since the days of King Henry VIII.

I think that Princess Margaret is a sort of supreme example did find it quite frustrating because I remember her saying once nobody ever asked her opinion about anything. She was just told when to turn up and told what to do, said Vickers.

Prince Andrew, younger brother of current heir-to-the-throne Prince Charles, has also had a tricky time with the media. Once viewed as a dashing former naval officer earned the nickname Randy Andy for his wooing of models and starlets, famously dating actress Koo Stark who had appeared in a soft-porn film.

His 1986 marriage to Sarah Ferguson ended in divorce in 1996, and since then the 55-year-old prince has endured much negative publicity with newspapers accusing him of enjoying a jet set lifestyle at taxpayers expense.

He has endured a torrent of unseemly headlines this year after a woman claimed in US court papers she was forced to have sex with him as an underage girl.

Unfortunately it seems that Prince Andrew hasnt really found quite the right role for himself, so I think theres an element of frustration in what hes trying to do, said Vickers, saying his hard work had been overshadowed by bad press.

The most recent royal to have the spare role was Williams brother, Prince Harry. He had a reputation as a royal wild child, dabbling with marijuana and under-age drinking as a 17-year-old, clashed with paparazzi outside nightclubs and wore a Nazi uniform to a costume party in 2005 which offended Jewish groups.

Before his second tour of duty in Afghanistan in 2012, the 30-year-old was pictured naked cavorting with an apparently nude woman at a party at a hotel in Las Vegas, summing up the problem he faced in a later apology. In Harrys case, his troubles have seen his popularity actually increase, seeming to resonate with the public.

He has his faults but by god he has got his good points and when I go on tour with Harry its a joy because every day he puts 110 percent into it, veteran royal photographer Arthur Edwards said.

He might be the spare to the heir but hes made himself very relevant.

Like Prince George, the worlds media set to lavish attention on William and Kates new baby when he or she is born, but commentators believe the couple, who they say have set out to be as down to earth as possible for royalty, are better placed than previous generations to help their child cope.