Former Good Morning Britain star Morgan had boasted about viewing figures for his new Uncensored show, which is airing on new global channel talkTV from Rupert Murdoch's News Corp.
However, McCoy – who announced in December that he was stepping down as a presenter on GB News – waded in and asked Morgan to prove it by showing the official data.
Morgan, 57, tweeted: “UPDATE: @PiersUncensored beat Sky News & GB News in the UK ratings AGAIN last night.. still very early days, but the show is already gaining real traction around the world, making lots of news, and as I hoped, annoying all the right people.”
McCoy then responded: “Please show the figures. #Facts.”
Morgan replied: “Show me yours. Oh wait…”
“Retirement, Piers. Try it,” said the screen veteran, 60.
But Morgan snapped back: “What, and be as bitter as you?
McCoy later told his followers on the site that Morgan had blocked him.
He also shared a screengrab, captioned: "Ooh.."
Fans flocked to Twitter to joke that McCoy had been censored.
But, in a nod to Morgan’s mission to cancel ‘cancel culture’, Coy teased: “Can’t have been.
"He says he’s open to debate...”
Morgan’s new show debuted in April and had a strong start.
It was reported that it brought in an average audience of 317,000 and peaked at around 400,000, beating the numbers of rivals BBC News and Sky News.
However, it has been reported that viewership fell after that.
According to Deadline, the second episode was viewed by an average of 216,000 people.
Watch: Piers Morgan bonds with climate activist who also walked off GMB