Peter Milner was locked up for 20 years after being found guilty of sexually abusing two girls while he lived in the Swansea area in the years before he moved to Shropshire.
Up until last Saturday, just two days before he was jailed, the 49-year-old was running coaching sessions at The Shrewsbury Club.
Milner, of Church Close, Shawbury, had not told the club he was subject to an ongoing court case and authorities had also failed to warn the club.
South Wales Police confirmed it was investigating Milner in 2016 in relation to allegations of historic sexual abuse and asked West Mercia Police to carry out safeguarding checks.
Both forces said the other should have carried out checks.
A South Wales force spokesman said: "South Wales Police began investigating Peter Andrew Milner for historical sexual assaults in 2016.
"Milner was not living in the South Wales Police force area, so in line with normal procedures, a request was made to his home force to carry out safeguarding checks in relation to his contact with children."
But West Mercia Police Chief Superintendent Tom Harding said: “Such management of Milner while under investigation would fall to the investigating force.
"At no time did South Wales Police inform us that Milner was acting as a coach.”
Dave Courteen, managing director of the Mosaic Group which manages the Shrewsbury Club, said: "We have been made aware of the conviction of Peter Milner who had worked at the club as a self-employed instructor teaching Jiu Jitsu.
"We were not previously aware of the allegations, nor of the court case and police investigation until its recent conclusion. And as a team we have been horrified to learn the nature of these historic crimes.
"The incidents occurred in Wales, long before he moved to the county, and there has been no suggestion of any inpropriety during his time with the club nor had any parents, children or other members raised any concerns about his behaviour.
"During the time that Mr Milner was at the club he underwent DBS checks as well as our own robust checks with nothing flagging.
"Our safeguarding procedures and practices meet or exceed all industry standards, however our senior team has chosen to complete a full review to highlight any additions or enhancements that we may be able to make in the future as part of our unequivocal commitment to the wellbeing of all of our members, of all ages.”
Milner began abusing his first victim when she was aged just five, with the attacks continuing until the girl reached puberty.
After each assault Milner would give the girl some money, or make threats about nobody believing her if she told anyone.
On one occasion, he sexually assaulted another girl in what was described in court as an "opportunistic" offence. Again he gave the youngster money afterwards.
There is no indication that he committed further crimes once he had moved to Shropshire.