By Clare Gabriel
BBC Wales News website
Princess Michael of Kent has revealed the secret of keeping a royal marriage alive - separate bathrooms and bedrooms.
The princess said modern British royals now marry for love
The princess's own marriage of 33 years to the Queen's cousin, Prince Michael, has been the subject of press attention in recent weeks.
But she told an audience at the Hay literary festival that she "got my prince and married him".
The princess is the first royal author at the festival, at Hay-on-Wye.
The 61-year-old princess, born Baroness Marie-Christine von Reibnitz, at Carlsbad in what is now the Czech Republic, said her mother gave her good advice when she married Prince Michael.
"My mother when I married said you must have separate bathrooms and separate bedrooms. That was the only sure way of keeping a marriage alive because it was an invitation as opposed to just being there and that makes it more romantic.
"I have been together with my husband for 33 years. Romance can still be there if you don't see each other brushing your teeth. There's something very nasty about brushing your teeth and then all that flossing."
The princess, Hay's first royal author, said she was in Wales to talk about her new historic work, Cupid and the King, published on 5 June, which details the roles of five royal paramours, including Nell Gwynne and Madame de Pompadour.
The princess consulted a medium in research for her next book
The book and her previous work, The Serpent and the Moon, tells what happened when kings were required to marry to ensure the future of their dynasty.
She said that was no longer the case, of course, in Britain where monarchs now marry for love.
But referring to her own marriage, she said that under the Royal Marriage Act, members of the British royal family were still banned from marrying Catholics.
She said: "I'm a Catholic, you know, not very good news!"
The princess told the audience that she did not look at the 20th Century in her investigation into royal love.
"Because you see this head on these shoulders, and there is also a place called 'the Tower'. I don't touch the 20th Century."
She also revealed that she was now looking at writing a novel set in 15th Century France, around the period of Joan of Arc.
"I'm having to make a novel because I can't find enough out about my heroine, who was murdered," said the princess.
She said she had consulted a medium who had given her clues about what the fate of her heroine had been.
Asked by a member of the audience whether her books were available in the libraries, Princess Michael said: "Hmmn! It only costs £10.99."