Transformers: Age of Extinction went on to become one of the biggest blockbusters of 2014, but uncertainties remain over Transformer 5, as director Micheal Bay has hinted at his exit from the second trilogy.

While fans remain concerned about the future of the movie, voice-over artist and actor Peter Cullen -- the original voice of Optimus Prime -- has revealed how he worries about Transformers becoming darker with each sequel.

Cullen shared his concerns over the image of the Transformers at the SacAnime2015 conference, Air Herald reported.

According to the website, Cullen has voiced his opinion before the studio and the writers, while hoping that the live action thriller returns to its original formula.

He also revealed that he refused to dub one dialogue for Optimus Prime as he though it wouldn't fit the Autobot leader's character.

Transformer 4 introduced a darker storyline and hit a jackpot at the box office, however, critics slammed it for over-the-top action sequences and lack of storytelling.

So much so that Razzie nominated the fan favourite film as the worst movie of 2014.

Recently, Mark Wahlberg refuted rumours that he was abandoning the fifth instalment of the highly popular franchise.

Rumours are abuzz that Jonathan Liebesman, the man who helmed Teenage Mutant Ninja Turtle, may direct the troubled project.

The much anticipated sequel may feature Optimus Prime's journey into outer space, while his army of Autobots will stay on earth to protect human kind from the Decepticons.

Transformers 5 is expected to be released in 2016.