Princess Margaret's Rolls Royce which she had tailored to her specifications is expected to fetch £55,000 as it goes under the hammer later this month.
Margaret owned the car from 1980 until her death in 2002, when it was inherited by her son David Linley, the Earl of Snowdon.
The 1980 Rolls-Royce Silver Wraith II was modified for her requirements, and carried guests like the Queen, Diana before she was married to Prince Charles, and former US president Ronald Reagan and First Lady Nancy.
It has 48,000 miles on the clock, and was used to take members of the Royal Family to her funeral.
Only 2,135 Silver Wraith IIs were made between 1977 and 1980.
The car is up for auction with H&H Classics at IWM Duxford in Cambridgeshire on 26 May.
Among the specifications that Margaret requested is a false floor, enlarged vanity mirrors and green upholstery throughout.
She also had the upholstery piping on the front and rear seats moved back so that it wouldn't be on her neck if she chose to recline.
The roof above the windscreen had to be modified to accommodate a flag mast and illuminated shield.
According to the auctioneers, the princess only gave Rolls Royce five months to make the changes, meaning some of the unique fittings she required were taken from her former car and then requisitioned for the newer one.
She used the pseudonym of "Mr Edgar" through the process.
The car was delivered to Kensington Palace on 16 May 1980.
Earl Snowdon sold the car after Margaret's death, which he was criticised for, but it's reported that he had offered the car to the Royal Mews before selling it.
Damian Jones, head of sales at H&H Classics said: "This is a very special, very important and low mileage Silver Wraith II which surely belongs in a museum.
"It is offered for sale with copy build records, service invoices and a wealth of period photos."
Watch: Susannah Constantine on Princess Margaret being her second mother