Sylvester Stallone spotted on Guardians of the Galaxy Vol. 2 set

He'd be set to join the likes of Kurt Russell, Elizabeth Debicki, Chris Sullivan, Pom Klementieff, and Tommy Flanagan

Looks like the cast of Guardians of the Galaxy Vol. 2 may be seriously bulking up.

Following news Kurt Russell, Elizabeth Debicki, The Knick's Chris Sullivan, Pom Klementieff, and Sons of Anarchy's Tommy Flanagan were all set to join director James Gunn's crew of misfits, rumours are now circulating that one more major addition has been made to the cast.

Gunn promised on Facebook, "we'll have more surprises for you in the coming months"; and no bigger surprise could there be than Sly Stallone himself clambering aboard.

New York Daily News is reporting that Stallone was recently photographed holding a script, leaving the Atlanta hotel where the film's cast is staying, and getting into the car usually assigned to star Chris Pratt.

There are coincidences, and then there's the inescapably obvious; though Stallone's role is entirely unconfirmed, a source to NY Daily News claims the actor will indeed be making a cameo in the upcoming sequel.

Though Russell was previously tagged by fans as the likely candidate to play Peter Quill (Pratt)'s father, expect plenty of focus now shifting to Stallone as to the identity of Papa Star-Lord.

If the rumours are true, Stallone will join original crew Pratt, Zoe Saldana, Dave Bautista, Vin Diesel, Karen Gillan, Michael Rooker, Sean Gunn, Glenn Close, and Bradley Cooper as the voice of Rocket.

Guardians of the Galaxy Vol. 2 opens 31 July.