There are many reasons not to like Starred Up. It’s excruciatingly violent, depressingly bleak and carries the threat of being Yet Another Slice Of UK Grim Pie. Oh goody. Please serve The Sloth an extra large portion. There are also many reasons to admire it. And they begin with Jack O’Connell.

Jack plays Eric, a violent young offender entering prison. Eric has evidently been ‘starred up’, the process whereby a young offender deemed too dangerous to be held in a juvenile detention centre is ‘promoted’ to adult prison. Distinguishing himself from the outset by brutally attacking his prison guards, Eric doesn’t just embrace violence, but appears to positively relish it.

It turns out Eric has a motive for joining the big boys.  His father, Neville (Ben Mendelsohn), is held in the same prison and is something of a Top Dog. Neville, naturally, is delighted to see his son following in his own aspirational footsteps. But there is another man viing for Eric’s attention. Middle-class, genial Oliver (Rupert Friend) is a prison social worker who runs a talking therapy group and doggedly believes even the most extreme offenders can be rehabilitated.

This is not a film for the squeamish. All the nastiest elements of prison life are laid bare: drugs, corruption, sexual predators, and the ever-present violence is raw, graphic and very difficult to watch. This could seem clichéd but, filmed inside a real prison, it captures the claustrophobia and constant, simmering tension with disconcerting realism. Best of all are the performances. Jack O’Connell inhabits Eric with such virulent abandon, menace and confused vulnerability we had to remind ourselves we were watching a fiction.

Starred Up is not a film you enjoy. It’s a film you emerge from disturbed, shaken and with a greater understanding of an institution most of us, fortunately, know little about.

UK release 21 March. Want something equally challenging? Try Under The Skin.