The 31-year-old shot to fame as part of the street dance troupe after winning the third series of BGT in 2009, beating Susan Boyle in the live final.
More recently, he appeared as a judge on ITV’s Dancing On Ice and presented Can’t Touch This on BBC One and Flirty Dancing on Channel 4.
He will join Amanda Holden, Alesha Dixon and David Walliams for the first semi-final, which is being filmed next week.
Cowell broke his back in a number of places earlier this month after falling off a new electric bike he was testing at his house in Malibu.
He was taken to hospital for treatment, which involved a number of procedures including having a metal rod put in his back.
Cowell said: “Ashley will be a fantastic edition to the panel. He knows the power of the show; and the responsibility of it.
“I couldn’t think of a better person to sit in my seat whilst I follow doctor’s orders. I hate that stupid bike!”
Banjo said: “As if 2020 couldn’t get any crazier! It’s super humbling and super exciting!
“I remember being on that stage last year and talking about how it’s been 10 years since we won, and how people should never tell you, you can’t do something.
“Walking on that stage as a 20-year-old street dancer from the streets of east London and now I am walking out there and stepping in for the big man!
“It feels really nice, almost poetic.”
This year’s BGT semi-finals were due to take place in May but will now be spread across five pre-recorded episodes.
The final will air in October, with the winner taking home £250,000 plus a slot at the Royal Variety Performance.
Some acts will take part via video link whilst others will perform in the TV studio without a live audience.
Instead, a virtual audience will watch from their homes and their reactions will be broadcast into the studio on a giant screen.
