A study of lesbians who have left long-term heterosexual relationships suggests that many may have switched sexual orientation in the middle of their lives.

Researcher Christan Moran, who interviewed the small sample of more than 200 women, believes this may be evidence that sexual orientation, at least for women, is more fluid than previously thought.

Ms Moran, from Southern Connecticut State University in the US, told the Daily Telegraph that there was “great potential for heterosexual women to experience a first same-sex attraction well into adulthood”.

She claimed that many women who came out in middle age or after marriage were wrongly dismissed as having been in the closet or having repressed their feelings.

Instead, Ms Moran argued, there was evidence that these women may have made “a full transition to a singular lesbian identity … in other words chang[ing] their sexual orientation”.

But she added that such women faced great difficulties in leaving the “undeniable privilege” of heterosexual marriage and that those with children were especially likely to struggle with their new identities as lesbians and the fear of society’s reaction.

In another study, Lisa Diamond, professor of psychology and gender studies at Utah University, tracked 100 women who declared some degree of attraction to the same sex over ten years.

At the beginning, most of the women saw themselves as lesbian or bisexual, with a few refusing to be labelled.

Ten years later, two-thirds of the women had changed the way they described themselves, with some bisexual women becoming lesbians and women who previously called themselves lesbians now identifying as straight.

One celebrity example of a woman leaving a heterosexual marriage to begin a relationship with a woman is Ally MacBeal actress Portia di Rossi. She divorced her husband in 1998 and is now married to Ellen DeGeneres.

Other women who have left straight relationships to come out in middle age are fashion guru Mary Portas, 49, who recently tied the knot with Grazia fashion writer Melanie Rickey, and psychologist Susie Orbach, 63, who is dating author Jeanette Winterson.