Police are searching for a man who escaped from custody after switching identities with his twin brother.
Simon MacLellan, 27, from Gosport, has not been seen since Friday afternoon when he was released by magistrates.
MacLellan was on remand at HMP Winchester in connection with a serious assault. His twin, Mark, is also on remand for a less serious offence.
Hampshire Police said a 27-year-old man had been arrested in connection with aiding and abetting an escape.
Mark MacLellan was due to appear at Fareham Magistrates' Court.
It is thought the pair - who are non-identical twins - switched identities in prison and the wrong brother was taken to court.
The mistake was spotted by prison officials shortly after 2100 BST on Friday.
'History of violence'
People are being advised not to approach Simon MacLellan if they see him as he could be violent.
"We are currently doing everything we can to find him and would strongly advise people if they see him not to approach him," said Detective Inspector Joanne Smith, from Fareham CID.
"McLellan has a history of violence and should not be approached under any circumstances."
McLellan is described as white, with brown hair and blue eyes.
He is 5ft 8in and slim and was last seen wearing a grey polo shirt with a small white stripe across the chest and blue jeans.
A statement released by the Ministry of Justice said: "A prisoner from HMP Winchester was mistakenly released instead of his brother.
"The prison are treating it as an escape and an internal investigation is under way.
"This is now a matter for the police."
MacLellan is due to stand trial in August in connection with an offence of grievous bodily harm following an assault in Gosport in December 2008.
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