Susanna, 46 – who presented today's GMB alongside Ben Shephard, 42 – asked Harriet, 66, why she hadn't brought up the whole thing sooner, after the politician opened up in her new memoir A Woman's Work.
In her book, Harriet explained how her former professor had said that he would "guarantee her a 2:1 if she slept with him", while she was studying at the University of York in the 1970s.
Susanna then grilled the former Labour deputy leader why she hadn't come clean about the alleged offer beforehand.
The presenter said: "Because you didn't complain about them at the time, one of them is now dead. They don't have a chance to defend themselves."
Harriet then replied: "He was like 20 years older than me, I was just a student. That's what it was like for a young woman then and the same happened when I was doing my legal training."
Susanna Reid grilled Harriet Harman on today's GMB
Harriet said that her former university lecturer offered her a better degree if she had sex with him
You didn't complain about them at the time and one of them is now dead. They don't have a chance to defend themselves.
Susanna Reid to Harriet Harman
She continued: "That issue of sex for grades is still a thing. You just felt horrified, but didn't think there was anything you could do about it. I've been disturbed by the fact that people thought somehow I would've made it up to sell my book. People criticise the victim, I'm 66, I've been a politician for 30 years…why would I need to make it up?"
Susanna went on to ask: "Was there a point in your career where you thought you could've made a complaint? When that person was around," as Harriet shared: "The idea of 'let's stop this for a moment' [didn't happen]. You don't want to make that an issue. You want to move on.
"It's because I was looking back at my life that I decided to put that in [the book]. I wasn't the only one who he did it to, a couple of years later another woman said he did the same to her."
Susanna then said: "You're adding another allegation there," as Harriet insisted: "It's in the book."
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Susanna asked Harriet why she hadn't said anything sooner
GMB: Piers Morgan & Susanna Reid take on politicians
Tue, January 24, 2017
Good Morning Britain hosts Piers Morgan and Susanna Reid talk politics with Theresa May, Nigel Farage, Donald Trump and more.
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Good Morning Britain talks politics with Theresa May, Donald Trump, Nigel Farage, & more
Viewers were quick to take to Twitter throughout the chat to accuse Susanna of being slightly harsh to Harriet.
One person posted: "@GMB @susannareid100 @benshephard disgraceful basically saying @HarrietHarman made up allegations of sexual harassment to sell her book."
"@GMB progress on reporting historical sexual crimes being undone by @susannareid100 grilling @HarrietHarman on why she never told," a second added, alongside a sad face emoji.
A third wrote: "@GMB @benshephard @susannareid100 Not impressed at the near victim-shaming of @HarrietHarman. Many don't report. That shouldn't be judged."
A further social networker said: "Sorry @GMB but how @susannareid100 just treated Harriet over historic abuse isnt fair! #NeverTooLateToTell."
Good Morning Britain airs weekdays at 6am on ITV.