Victor Nealon, 53, was convicted of attacking a woman outside a nightclub in Redditch in 1996.
He asked the Criminal Cases Review Commission (CCRC) to examine his case but was turned down twice.  His conviction was quashed last year.
The commission has now said it should have investigated more thoroughly.
File on 4
Commission chairman Richard Foster said: "I regret the fact in this particular case we missed something and I apologise to all concerned for the fact we did so."
Mr Nealon, who had always denied attempted rape, was arrested after a woman was sexually assaulted on her way home from a nightclub in Redditch.
The postman was convicted after a trial at Hereford Crown Court and given a life term.
Mr Nealon said he wanted the CCRC to get more information about the forensic evidence presented in the prosecution's case.
He said the CCRC requested the information from West Mercia Police but failed to ask more questions when the force said a file of evidence had been lost.
"I depended on people in that position to research paperwork and they didn't do it," Mr Nealon said.
Mr Nealon's defence team eventually discovered an unknown person's DNA on clothing that had not been disclosed by West Mercia Police. His conviction was finally quashed in 2013.
He said: "I could have been out at least ten to 12 years ago but, on account of the CCRC and their failure to research a paper trail, I remained in prison."
West Mercia Police said it was studying the Court of Appeal's full judgement which would form part of the ongoing review of the case.
Find out more about this story on File on 4 on Tuesday 20 May at 20:00 BST on BBC Radio 4.
A man who spent 17 years behind bars wrongfully convicted of attempted rape has received an apology from a body set up to examine miscarriages of justice.