A 51-year-old man from Jersey is trying to prove he is the illegitimate son of Princess Margaret, making him potentially 12th in line to the throne.
Robert Brown wants access to Princess Margaret's will
Robert Brown is asking the High Court in London for access to the wills of the Queen Mother and Princess Margaret.
In a summons, he cites the Queen, the Attorney General and the executors of the wills of the Queen Mother and Princess Margaret as defendants.
The Queen and her representatives have declined to comment about the case.
Mr Brown has also tried, unsuccessfully, to get information from the Department for Constitutional Affairs as well as the Prime Minister's and Cabinet Office.
He claims the case raises the major constitutional issue of the rights of illegitimate children in the line of succession.
If Mr Brown was the son of the late Princess Margaret, he could become 12th in line to the throne - between Zara Phillips and Princess Margaret's only known son Viscount Linley.
Mr Brown insists there is a public interest issue on the grounds of openness and consistency and says he would not object to the wills being made public, if they had once been made private to protect an illegitimate child.
The qualified accountant, who stood for election in Jersey in 2004, has been trying for four years to prove his parentage.
He told BBC News: "I am not going to give up. I'm driven by a host of memories and a fundamental need to know.
"I have some evidence, including a photograph taken in October 1956 of a child sitting in a car in Kenya. That child was me and I think the photo was taken by Princess Margaret."
Mr Brown says there is some mystery surrounding his birth, with a Times announcement giving the date as 5 January 1955, although his birth was not registered until 2 February and the certificate was dated 4 June.
Mr Brown says he has been trying for years to prove his parentage
His parents are registered as Cynthia and Douglas Brown and Mr Brown claims Cynthia Brown had been a model working for Hardy Amies, who was a favoured designer of the princess.
He says years of research has led him to firmly believe the princess was forced to give up her "secret" son, but did visit him when he was young.
He claims there are other things to back up his claim, including a mystery Privy Council meeting on the day he was born.
He claims there were rumours of an illegitimate child in High Court circles with the closed wills of both the Queen Mother and Princess Margaret.
"The point is they've never, ever admitted or denied it", he said.
"This court summons is one avenue of many which is achievable, clean and clear cut, but win or lose, I won't give up."
It is not known when it will go before the High Court's Probate Division.