The presenter told viewers this morning: “Some of you may cheer, and others will boo” at his departure, adding that he was an “opinionated” and “disruptive” broadcaster.
The news followed widespread criticism over Mr Morgan’s comments on Prince Harry and Meghan Markle’s highly-publicised interview with Oprah.
During the interview, which was first aired in the United States on Monday, Ms Markle revealed that she had felt suicidal because of abuse she received for marrying into the royal family.
On Tuesday morning, Mr Morgan said: “I’m sorry, I don’t believe a word she said, Meghan Markle. I wouldn’t believe it if she read me a weather report.”
Ms Reid told him that his comments were a “pathetic reaction to someone who has expressed those thoughts.”
It later emerged that Ofcom, the UK’s broadcast regulator, had launched an investigation into Monday’s episode of GMB after receiving more than 40,000 complaints from viewers.
In an on-air statement on Wednesday morning, Ms Reid said: “Piers and I have disagreed on many things, and that dynamic was one of the things that viewers loved about the programme.
“He is without doubt an outspoken, challenging, opinionated, disruptive broadcaster.
“He has many critics and he has many fans.
“You all know that I disagreed with him about Meghan’s interview,” she continued, “and he himself clarified his own comments about her mental health on the show yesterday.
“There are many voices on GMB, and everyone has their say. But now Piers has decided to leave the programme.
“Some of you may cheer, and others will boo.”
She added: “He has been my presenting partner - Monday to Wednesday - for more than five years, and during Brexit, and the pandemic, and other issues, he has been a voice for many of you, and a voice that many of you railed against.
“It is certainly going to be very different, but shows go on - and so, on we go.”