Labour’s former deputy leader said she rejected the advance by her course tutor, Professor TV Sathyamurthy, saying she was “repulsed by him”.
The academic, who died in 1998, was said to have told her she was a “borderline” student who might get either a 2:1 or 2:2 but he would ensure she was awarded the higher grade if she slept with him.
But Carole Satyamurti, 77, said the university lecturer, who she separated from in 1972, was “very respectful of women”.
The retired poet and sociology lecturer told the Daily Mail: “That’s horrible. I find this very hard to believe. The story sounds very dubious to me.
“He was very brilliant and full of jokes but very respectful of women. I’m sure he wouldn’t have done that.”
York University, where she was studying at the time, said her claims were “extremely concerning” and would constitute “gross misconduct”.
Ms Harman makes the allegation in her forthcoming memoir, A Woman’s Work, which will be published next week.
The Labour MP, who graduated in 1972 aged 21, said she still gained a 2:1 in politics, despite rejecting his advance.
York University’s registrar and secretary, Dr David Duncan, said: “The allegations made by Ms Harman are extremely concerning.
“The University of York strongly condemns all forms of sexual harassment and will not tolerate it among staff or students.
“Behaviour of the sort described by Ms Harman would constitute gross misconduct and would lead to dismissal.
“We have an extensive support system in place for students and staff who are concerned about harassment of any kind.”