Jail staff appear to be too scared to break up the scrap between the two armed robbers - each serving seven years, according to a source.
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The brutal battle on 'G Wing' in HMP Onley was only broken up when other lags stepped in after the bust-up escalated.
The fighters were from rival London gangs in Peckham and Hackney and were trying to settle the score for top spot inside the nick, according to a source.
The melee was filmed by another lag and sent to a friend through WhatsApp.
A jail source said fights are common and staff can't control the inmates. He also says the harsh repercussions will be felt in the related gangs on the outside.