A cousin of the Queen who spent 55 years of her life being cared for at the Royal Earlswood Hospital, in Redhill, has died.

Katherine Bowes-Lyon died, aged 87, on February 23. A private family funeral was held.

It is believed Miss Bowes-Lyon lived at a care home in Surrey since the closure of the Royal Earlswood Hospital in 1997.

She began her residency at the hospital – now converted to luxury apartments – in 1941, when she was 14.

Miss Bowes-Lyon and her sister, Nerissa, were two of the daughters of John Herbert Bowes-Lyon and his wife Fenella. As John was the brother of Elizabeth Bowes-Lyon, the Queen Mother, the two daughters were first cousins of Queen Elizabeth II, sharing one pair of grandparents – Claude Bowes-Lyon, 14th Earl of Strathmore and Kinghorne, and Nina Bowes-Lyon, Countess of Strathmore and Kinghorne.

The Royal Earlswood Hospital provided accommodation for some 230 men and women affected by mental illness at the time of its closure.

Katherine Bowes-Lyon was placed in the hospital, in Asylum Arch Road, Earlswood Common, along with her sister Nerissa and three of their cousins – Idonea, Ethelreda and Rosemary Fane – during the Second World War.

Nerissa died on January 22, 1986 aged 66. She was buried in a humble plot at Redstone Cemetery in Philanthropic Road, Redhill.

In 1996, the Daily Express reported that the Queen Mother only learned that Katherine and Nerissa were in the Royal Earlswood in 1982, when the hospital's league of friends wrote to her.

The newspaper reported that the Queen Mother sent a four-figure sum of money so birthday and Christmas presents could be given annually to the cousins.

The hospital closed as a result of a "care in the community" programme, which sought to place institutionalised residents back in the community, but with special help available to assist them in coping.

In 2012, Channel 4 broadcast a controversial documentary emotively entitled The Queen's Hidden Cousins.

It suggested the Royal family had behaved uncaringly towards the daughters of the late John and Fenella Bowes-Lyon, who were the older brother and sister-in-law of Queen Elizabeth, the Queen Mother. It is understood the monarch was "hurt" by the programme as, according to press reports and in reality, the Royal family had supported the sisters.

The programme also stated that John Bowes-Lyon never visited his daughters as they grew up. But the Daily Telegraph reported last month that the programme failed to mention that John Bowes-Lyon had died in 1930.

A reporter from The Sun newspaper once tried to get into the hospital to speak to Miss Bowes-Lyon, while carrying a birthday bouquet of flowers for her.

And there was outrage three decades ago when an American TV film company went to the Royal Earlswood Hospital and secretly filmed Miss Bowes-Lyon inside.

Interviewer Geraldo Rivera rigged up a camera in a shoulder bag and filmed her.

Burke's Peerage recorded in 1962 that Katherine and Nerissa Bowes-Lyon were dead. It was only in 1987 that they were "discovered" living at the Royal Earlswood, founded in 1853 as the National Asylum For Idiots.

Burke's – the "Facebook" of its day – claimed there had been no attempt to cover up their existence and said the mistake had been caused by errors made by a family member when filling in the forms they had been sent.

The Royal Earlswood's first residents moved in on April 15, 1855, and the official opening day was July 5 that year. Prince Albert, husband of Queen Victoria, officiated.

An exhibition of the hospital memorabilia can be seen at a museum in Teddington, Middlesex.