Dr Qazi Rahman, of the University of East London, said the reaction of the lesbian test subjects was closer to that which would be expected among straight men. The same was true among gay men, although to a lesser extent, in that their reaction was closer to that of women.

Dr Rahman said the results could affect the way society dealt with sexuality and issues of sexual orientation.

Along with Dr Veena Kumari and Dr Glenn Wilson of the Institute of Psychiatry, part of King’s College London, Dr Rahman claimed their study offered the first independent evidence of a non-learned, neurological basis for sexual orientation.

Using a technique called prepulse inhibition (PPI), the test subjects were unexpectedly startled by loud noises preceded by a quieter noise.

Using electrodes on the face just beneath the eye, Dr Rahman measured the strength of the involuntary eye-blink responses to the noises and compared them with those to a loud noise on its own – the lower the response, the stronger the level of inhibition.

Heterosexual men had a PPI of 40%, compared to 13% for heterosexual women. Lesbians had a PPI of 33% – significantly stronger that that of straight women. Gay men averaged a PPI of 32% – slightly lower than straight men.

Dr Rahman said: “The startle response is pre-conscious and cannot be learned. It is mediated by an ancient region of the brain called the limbic system which also controls sexual behaviour. This is very strong evidence that female sexual orientation at least may be ‘hard-wired’ in this region.”