As movie transformations go, this is one of the best we've ever seen - and if we didn't know who this leading man was we don't think we could guess.

Have a look at the battered-looking hunk in the still from new boxing flick Southpaw and see if you can work it out.

Now if you said Jake Gyllenhaal then well done, and take a gold star.

The 33-year-old star of Brokeback Mountain, Donnie Darko and Source Code was beefed up with the aid of a strict workout regime for his role as a troubled boxer.

And his transformation is especially startling considering the production followed Nightcrawler, for which he lost a ton of weight and looked super-scrawny.

Director Antoine Fuqua paid tribute to the star, saying: "Jake is going to change how people see him. I had him training twice a day in the boxing ring, seven days a week.

"I pretty much had him with me and my trainer every day. I took him to almost every fight. I had him train at Floyd Mayweather's gym in Vegas and we watched Floyd's fights, and the Manny Pacquiao fight.

"He trained in New York at Church Gym with real fighters. We literally turned him into a beast… Jake, my god, he's a very electric, powerful fighter in this movie, and a guy who fights for his daughter.

"I'm confident that this will change how people see Jake as a leading man."

We think it already has!

The movie, which has no UK release date yet, also stars Rachel McAdams, Naomie Harris, Forest Whitaker, Victor Ortiz, Tyrese Gibson and Miguel Gomez.