The 2022 Festival will run from August 12 to 14.
The quartet had been playing in the summer event outside St Myllin's Church in the town for half a century.
Founded in 1953 they were the longest serving chamber music ensemble until it disbanded.
The first concerts held by the original quarter developed into was is now the annual Llanfyllin Music Festival.
A spokesperson for the music festival said: "While the Allegri Quartet has now disbanded its first concerts paved the way for the festival with its wide variety of musicians and international artists."
Tickets for the event are available from the Llanfyllin Music Festival's website.