Mark Ruffalo accidentally let slip the titled for 'Avengers 4'.

The 50-year-old actor stars as Bruce Banner and his alter ego The Hulk in the Marvel Comics Universe ensemble movie franchise, and during an appearance on 'The Tonight Show starring Jimmy Fallon', he seemed to make a huge slip up when he accidentally blurted out the subtitle for the upcoming fourth instalment.

During his interview, the entire audio clip - which saw him unveil key secrets about the plot, as well as the true title - was bleeped out, both in the studio and on television, but fans have been doing their best to lip read what the actor was saying.

It appears the titled could be 'Avengers: Annihilation', which also fits with an earlier tease that the film's directors, The Russo Brothers, had put out in which they said the title would include two A's.

After his on-screen blunder - which was set up as a joke for the show - Mark took to Twitter to beg show host Jimmy not to include the spoilers in the final programme.

He wrote: ''I trust that you will cut my spoiler slip on the show tonight. That was ''off the record'' homey. Please don't get me in trouble with Marvel (Barry) again. DM me back. (sic)''

To which Jimmy replied: ''Dude, it's too late - already sent the final show to Burbank for air. Are you sure you're gonna be in trouble? We could maybe bleep it out. What is ''DM''?''

Mark has a history of letting slip some fairly major secrets, after he previously spoiled the ending of 'Avengers: Infinity War' a year before its release, and live-streamed the first 15 minutes of 'Thor: Ragnarok' at its premiere.

'Avengers 4' is set for a 2019 release, and Chris Evans - who plays Captain America - recently confirmed the movie had wrapped filming, as he hinted that he'd made his final outing as the superhero after eight years in the role.

He tweeted: ''Officially wrapped on Avengers 4. It was an emotional day to say the least. Playing this role over the last 8 years has been an honor. To everyone in front of the camera, behind the camera, and in the audience, thank you for the memories! Eternally grateful.''