The 43-year-old journalist's ongoing feud with Piers, 55, has raged on during lockdown, and after Morgan's comments about his interview with Health Secretary Matt Hancock on Tuesday, Walker hit back.
Dan had taken to his Twitter page to let viewers know business minister Alok Sharma would be appearing on the show, to which Piers found hysterical.
Piers made a dig at the BBC presenter, saying in a tweet to Walker: "Sharma... This will be fun. You warm him up with your Govt-approved soft b***s Daniel & we'll follow up on @GMB with the real questions."
Dan didn't hold back as he shot back an insult to Morgan, writing: "You keep up your pound shop political Googlebox and try not to let all those toys fall out of the pram at once.
"See you nice and early."
Dan's tweet comes after Piers slammed the BBC Breakfast host for "blowing" his interview with Health Secretary.
As he presented GMB, he watched the BBC programme and remarked that his rival Dan was spurting "complete nonsense".
Piers said: "Get in there Walker, for goodness sake man. He's talking complete nonsense!"
Co-host Susanna interjected, in a bid to calm things down, saying: "Oh my goodness. Now you're ranting at presenters on other programmes."
"What could be a more important bit of data than how many people are being tested? What could possibly be more important.
"And yet you pretend it's meaningless," Piers added.
Good Morning Britain airs weekdays at 6am on ITV