Premiership football is one of the most glamorous sports in the world, beamed to hundreds of millions worldwide and worth over £20 billion a year.

But there's a darker element lurking close to the surface. In recent years football hooliganism has been a secret network of clashes between rival firms, planned well away from stadiums and CCTV.

But last season things changed. Violence is now breaking out inside the ground, on our streets and even fighting on our transport network.

Police claim arrests for disorder are down, but football's European governing body warns of a return to hooliganism's dark days.

In this film we follow some of the most active youth firms in the country, home and away, to see just how far the rules of football hooliganism have changed. Show less