Piers, 55, and Eamonn, 61, are good friends now, but had a fall out two decades ago when Piers was the editor of The Mirror.
Eamonn had been left very unimpressed when Piers published photos of Eamonn playing gold with the headline "Telly-tubby", in a reference to his weight.
The GMB star addressed their spat in his latest column for the Mail, as he revealed that Eamonn is now his fitness inspiration and he wants to weigh the same as him.
He wrote: "My ITV breakfast show predecessor Eamonn Holmes and I are now good friends, but we fell out badly 20 years ago when I was editor of the Daily Mirror.
"To his great credit, Eamonn has since dropped from 18st to 15st through juicing, fasting, walking and Pilates after making a New Year's resolution five years ago to get slimmer and healthier."
Eamonn had previously said that his weight gain was down to his early morning shifts which sparked his bad eating habits while presenting GMTV.
Piers continued: "My own Good Morning Britain day regime is usually porridge and honey at 5.30am, Marmite on toast at 7.30am, eggs and something when I get home at 9.15am, possibly another sandwich snack at 11am, and a hearty lunch at 12.30pm before attacking the biscuit tin in the afternoon, until my wife Celia locks it away.
"And the former 'Telly-Tubby' has gone from being the object of my merciless fat-mockery to my fitness role model.
"So my resolution is to weigh the same as Eamonn Holmes by the end of 2021."
This comes after Eamonn admitted that he struggles to look at pictures of himself from his days hosting GMTV, as he weighed three stone heavier than he does today.
He told Daily Star: "I didn’t realise how much it changed my life, your habit and how you are.
"I was three stone heavier when I was on GMTV than I am now.
"The pictures of me in my 30s and 40s, I just can’t look at."
GMB continues on Monday at 6am on ITV and This Morning resumes at 10am on the same channel.