A man posing as a lawyer managed to enter a maximum security prison in Colombia and kill one of the jailed leaders of the now defunct Cali drugs cartel.

Prison officials said the gunman approached Helmer Herrera as he was playing football in the exercise yard, hugged him, and then shot him in the head.

Fellow prisoners jumped on the man and beat him unconscious.

Correspondents say Herrera was one of the top five leaders of the Cali gang, which once controlled some eighty per cent of the world's cocaine trade.

The motive for his murder was not immediately clear; reports say the gunman, who is now in hospital, was drunk.