Local residents at Usk, Gwent, complained about the behaviour of the roving buccaneers who drank at the bar and roamed around the carnival field collecting money for charity in buckets.
One mother said: 'They were just going wild - drinking, shouting and jumping around. It was very frightening because they were dressed as pirates and asking for money in these buckets. It is totally unsuitable for prisoners to be taking part in a fun day for families.'
Nick Evans, governor of Usk open prison, yesterday promised an investigation. 'Some people have said the men were over the top,' he said. 'It was an error of judgement in allowing them to collect money for charity.
'I'm sorry about that. There was supervision, but whether it was at the right level is something I'm looking into. I'm also looking into the boisterous behaviour of some of the prisoners.'
The men are serving sentences for a variety of offences, but none is judged to be a danger to the public. They were allowed to enter a float in the carnival. 'They were certainly not marauding prisoners accosting young girls or mugging old ladies,' Mr Evans said.