Royal fans who were excited for the return of The Crown might be shocked to learn the harrowing true story of the Queen’s cousins.

The "hidden" family members were confined to an asylum and registered as being dead after they were born with severe learning difficulties.

Daughters of John Bowes-Lyon, the Queen Mother’s brother, Nerissa Bowes Lyon was born in 1919, and sister Katherine in 1926.

The sisters were born with a mental condition that meant they were unable to communicate – something that their father feared would threaten their social standing.

At ages 15 and 22, Katherine and Nerissa – the Queen Mother's nieces – were hidden away in the Royal Earlswood Institution for Mental Defectives in Redhill, Surrey.

Nerissa Bowes-Lyon was placed in an asylum with her sister

Katherine and Nerissa were secretly placed in an institution that was cruelly dubbed The National Asylum for Idiots and hidden from the world.

Their admission was deemed necessary in 1923, when their Aunt Elizabeth – the Queen’s mum – married the future king, George VI.

They remained there for the majority of their lives and, according to reports, were rarely, if ever, visited.

Nerissa passed away aged 66 in 1986 and Katherine died six years ago aged 87.

Their story only came to light after Nerissa’s death when she was buried in a grave only marked by a serial number and plastic name tag.

Katherine Bowes-Lyon was kept at the institution after her sister's death

According to reports, the Queen Mother knew that they were not really dead despite being registered as deceased.

But she never visited either of them, and saw no contradiction in her patronage of Mencap, which campaigns against families placing their mentally challenged relations in state care, Mail Online reported.

The tragic true story is retold in the latest series of the Netflix drama.

In one scene, the pair can be seen watching on adoringly as their cousin – the Queen – arrives at the Royal Variety performance.

Cuddling dolls, they honourably stand for the national anthem, and salute their monarch, before being given pills by a nurse.

Later, Princess Margaret – played by Helena Bonham Carter – flies into a rage at their harrowing treatment.

Helena Bonham Carter returns as Princess Margaret this weekend

She fumes to the Queen Mother – who is played by Marion Bailey – “It's wicked and it's cold-hearted and it's cruel and it's entirely in keeping with the ruthlessness which I myself have experienced in this family.”

The Crown will of course be using artistic licence in the fictional retelling.

Writers claim that the Queen accepted the lie that her cousins were dead and not being hidden away.

It is not known when she discovered the truth, but following a Channel 4 programme in 2012 she was reportedly upset by suggestions that the pair had been abandoned.

And it is unknown when Margaret found out about the situation, or if she flew into a rage.

The Royal Earlswood was closed in 1997.