An author who has researched the history of Thailand's transgender community is to publish his theories in a book called The Third Sex.

Richard Totman, 53, has spent four years investigating the country's so-called ladyboys, and concludes they represent a sex distinct from men and women.

Mr Totman, a fellow of Oxford University, believes the third sex has been marginalised and stamped out throughout history.

Mr Totman, who lives in Haywards Health, began his research after a visit to Thailand in 1998.

He became intrigued by the lives of the ladyboys, who are born male but choose to live as females, often undergoing sex-change operations.

He revisited the country sporadically for four years, interviewing 43 ladyboys and several doctors. He also researched the history of the phenomenon, finding references in Hindu and Buddhist texts hundreds of years old.

He said: "In Thailand there is a substantial minority who do not fit into either the male or female category. What fascinated me was they were so ubiquitous and in the most part accepted as part of the cultural furniture. This acceptance is not prominent in the west."

Mr Totman believes because religions such as Hinduism are more accepting of transgender, the ladyboy culture has been allowed to grow.

It has grown as far as Brighton - The Lady Boys of Bangkok are performing in a marquee in Victoria Gardens, Grand Parade, as part of the Brighton Festival.

The performers spend hours to achieve elaborate feminine hairstyles and make-up.

Mr Totman said: "There is reference to third-sex individuals in many Asian religions. One image of a Hindu Buddha is half female and half male."

Dominant Christian and Jewish influences in the west, he claims, have historically persecuted the third sex.

He said: "The question is whether the third sex has been suppressed by religious dogma. It seems to have been permitted in Hindu and Buddhist-based cultures, but not in Jewish-Christian countries, where incoming colonial powers attempted to stamp it out."

Transsexual Mel Cherriman is chairwoman of the Clare Project self-help group for people with gender issues, based in Brighton and Hove. She believes there is more to gender than man and woman.

She said: "A lot of transgender people are aware from a very young age they do not conform to the stereotypes of male and female. Many grow up feeling they do not fit in and many people have been told trans-sexuality is impossible."

The Third Sex goes on sale on Thursday, May 15.

The Lady Boys of Bangkok can be seen in Brighton until June 15.