STOCKHOLM (Reuters) - The British-born Swedish princess whose secret 33-year romance with her royal husband became Sweden's best-known love story, died on Sunday at the age of 97, the court said.

Princess Lilian was the commoner wife of Prince Bertil, who died in 1997. They met and fell in love in London during World War Two, but had to keep their relationship secret for decades for the sake of the crown and to avoid a constitutional crisis.

The royal court said in a statement that the princess, born Lilian Davies in Swansea in August 1915, died peacefully in her sleep in the afternoon at her home in Stockholm.

She and Prince Bertil had to keep their love secret as Bertil's elder brother and heir to the throne, Prince Gustaf Adolf, had died in a plane crash in 1947 while the next brother, Sigvard, waived his right to the throne by marrying a commoner.

That left Bertil next in line until his infant nephew, Crown Prince Carl Gustaf came of age. If Prince Bertil had married a commoner he would have had to renounce his right to the throne, probably sparking a constitutional crisis.

It was not until after the crown prince became king in 1973, and married a few years later, that Prince Bertil and Lilian could finally get married themselves and appear in public.

(Reporting by Patrick Lannin; Editing by Stephen Powell)