Oppenheimer will be Christopher Nolan’s first R-rated film in more than 20 years

Christopher Nolan's upcoming film Oppenheimer has been given a surprise rating in something that hasn't happened in a while for the director.

The upcoming biopic thriller is based on the 2005 novel American Prometheus by Kai Bird and Martin J. Sherwin.

It will be Nolan's first R-rated flick since 2002's Insomnia.

This means that anyone under the age of 17 will require a parent or adult guardian to accompany them if they want to see the film in cinemas.

Nolan's Memento, released in 2000, also received an R rating in the US, however 2008’s The Dark Knight and 2011’s Inception have all been rated PG-13.

The film has an A-lister cast, with Cillian Murphy portraying real-life physicist J. Robert Oppenheimer, who is working alongside a team of scientists to build the world's first atomic bomb during World War II.

Emily Blunt, who plays Oppenheimer's wife, Kitty, will also be joined by Matt Damon, Florence Pugh, Robert Downey Jr., Rami Malek and Jack Quaid.

The film is slated for a July 21 release, and the official trailer dropped earlier this year.

Fans were blown away by the teaser clip and said it already has the makings to be incredible.

One fan wrote: "I don't expect less than an Academy Award nomination for Cillian Murphy as Best Actor and a nomination for Robert Downey Jr. as Best Supporting Actor. This movie is gonna be insane! Christopher Nolan never disappoints."

Universal Pictures

Another said: "Looks great, to be honest, I'm more surprised that they've never made a movie about this man's life before. It's hard to overstate how important his work has been in the grander context of human history."

While a third marveled: "Nolan, a well-calculating filmmaker who never disappoints."

The film marks the first biopic Nolan has directed; however, it's not the first time he's worked with Murphy, who has appeared in his Batman trilogy, Dunkirk and Inception.

And many predict that the film will finally score the Peaky Blinders actor his first Academy Award.

While speaking to Variety, the actor admitted he bore a 'huge responsibility' upon accepting the role.

“You realize this is a huge responsibility. He was complicated and contradictory and so iconic,” Murphy said of the character."

But despite this, he was confident working alongside his longtime collaborator.

"But you know you’re with one of the great directors of all time. I felt confident going into it with Chris.

"He’s had a profound impact on my life, creatively and professionally. He’s offered me very interesting roles and I’ve found all of them really challenging. And I just love being on his sets.”