The change is revealed in The Human Voice by Anne Karpf, which details research indicating the change. It shows that when 1945 recordings of women aged between 18 and 25 were compared with similar recordings from 1993, the average pitch of the later group was about 23 hertz lower — roughly equivalent to a semitone drop.

Singing coaches and audio archivists confirm the trend. Jonnie Robinson, a curator at the British Library who specialises in dialects, said: “Women’s voices do seem to have lowered over the last 50 years.

“Women have been striving to attain acceptance in a previously male-dominated society and they may have lowered their tone to enter that realm. A deeper voice might be associated more with power.”

Some women television correspondents say they have been encouraged to lower the tone of their voices. Jon Snow, the Channel 4 broadcaster, once said that “a woman without bass registers in her voice would find it very hard to get on in broadcasting unless she was exceptionally beautiful”.

Linda McDougall, Cherie Blair’s biographer, who has written extensively on women at Westminster, said a squeaky voice was a drawback in the corridors of power.

“If you speak in a high-pitched voice you should do something about it, in the same way you might put on make-up,” she said. Baroness Thatcher smoothed her shrill tones with coaching, while actresses such as Kathleen Turner, star of Prizzi’s Honor and Romancing the Stone, have perfected the deep voice on screen.

Francis Newton, a voice trainer from Yorkshire, said she had noticed the change in women’s voices but said there may be a physical explanation.

“One reason women’s voices have become lower is changes to their vocal chords. Now women are becoming larger the cords are longer, which means frequency or pitch is lower.”

Trinny Woodall, who co-presents the television fashion show What Not to Wear with Susannah Constantine, has a relatively deep voice but said last week that she had never seen it as an advantage. “I’ve never felt as though I was being taken more seriously because of the pitch of my voice,” she said.

Clive Upton, professor of English at Leeds University, linked change to the evolution of sexual roles. “Women had a larger child-rearing role so they needed to speak more clearly, and were judged more by their appearance and presentation than their profession,” he said.

“Now those differences don’t need to be signalled. Vernacular and idiom have been evened out between the sexes, and so might pitch and tone.”

According to Karpf, the pressure for women to imitate the husky voices of actresses and presenters was first observed in the 1970s. According to one academic study, women were tending to pitch their voices lower than advisable — around two-thirds of an octave higher than men’s, rather than the more usual one octave. Restricting the range of their voices could, it claimed, make them less expressive and risk injury.

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