Martin Scorsese used a prop in Shutter Island to give away the famous plot twist

Shutter Island is one of those films which is known for its iconic plot twist - but did you know there was actually a subtle clue from the very beginning that gives it away?

Safe to say you'll kick yourself when you see it.

The 2010 flick starring Leonardo DiCaprio and Mark Ruffalo follows Teddy Daniels (DiCaprio), a US Marshall who is sent to an institution for individuals deemed criminally insane to investigate a missing patient.

Though a few flashbacks drop hints about Teddy's history, it's not until the end of the film that the US Marshall learns he's actually a patient himself; one who was sent there after murdering his wife after she had killed their children.

It turns out that the whole 'missing patient' thing in the film was something Teddy came up with to help him deal with the trauma of losing his family and in a last-ditch attempt to help him remember who he was, the staff decide to go along with his detective delusions to see if it will help him accept reality.

Now, there has been a host of theories surrounding the Martin Scorsese film, but the director let eagle-eyed fans know the twist way before it was revealed all with a simple prop.

It’s been 14 years, and no one can get over its ending, but if we’d all paid a little more attention to DiCaprio’s hands, we wouldn’t have felt the blow as hard as we did.

The shocking ending to the film was right in front of us the whole time. (Paramount Pictures)

As you carry on through the movie, you start to realise some things, even though the prop was right there at the beginning.

For instance, Andrew Laeddis is an anagram of Edward Daniels, the name of Teddy, who is hunting for answers.

It’s all there for fans to figure out, but only if they’re looking.

Anyway, the prop was probably the biggest indicator as it sparked theories about what patients were and were not allowed to carry.

The answer? Lighters.

Do you remember that the detectives enjoyed smoking a lot?

But when push came to shove, Teddy never had a lighter and always had someone else light it for him?

Bingo. That’s the clue.

The film left audiences in shock at the ending. Paramount Pictures

After finding out who Teddy really is, and re-watching the film, viewers clocked it almost immediately.

Movie fanatic @imeugeneclark uploaded a video which explained that a prop used in the film was the key all along.

"Did you know that in Shutter Island a small detail reveals Teddy's true identity early on?" they explain.

"Of course, a big plot twist of the movie is finding out that Teddy Daniels isn't in fact a US Marshal but actually Andrew Laeddis, a patient at the mental sanatorium he is investigating.

"This twist is actually given away as every time that Teddy goes to smoke a cigarette, someone else lights it for him.

"This is because patients on Shutter Island are not permitted to carry lighters around with them."

Other fans pointed out more clues, such as his 'partner' Chuck (Ruffalo) struggles to take off his holster before entering the facility.

That’s because he doesn’t typically carry one and he isn’t a detective either.

"It's one of the earliest clues that his partner isn't who he says he is, since someone with his supposed background shouldn't be struggling with such a simple action," they wrote on Reddit.

What other things do you think told the truth about Teddy in the movie before it was revealed?