David Jamieson, the region's Labour commissioner, said Brexit was "good news for criminals" as he attacked changes to police systems for snaring crooks who had fled the area.
The avid Remainer complained to a meeting of his strategic crime board that the new system was too slow and allowed criminals to "potentially fall through the cracks".
He has been accused of using Brexit as a "scapegoat" for his own failure to hold West Midlands Police to account.
Having left the bloc Britain forfeited its membership of Europol, Eurojust and the European Arrest Warrant (EAW), and will no longer have access to EU databases which include criminal records, fingerprint and vehicle registration data.
Speaking at the meeting, West Midlands Police Chief Constable Dave Thompson said the force had 51 EAWs in place on January 1, all but six of which had extradition notices.
He said the National Extradition Unit (NEU) has taken over the management of 45 cases and was working with Interpol to facilitate arrests.
Mr Thompson said: "It is quite a new process and we can't possibly comment on how quick or how speedy it is in comparison to the European Arrest Warrant.
"But we do have good structures in place to ensure that we identify those that are outstanding. We do work through Interpol, we do work with the National Extradition Unit, and we will then see what the impact of the new procedures will be as time goes by."
Mr Jamieson, who is due to step down in May, said of the new measures: "This is hardly taking back control. The UK is left with a system that is slower and more time consuming, but worse still allowing criminals to potentially fall through the cracks.
"It is clear that Brexit has been good news for criminals trying to avoid justice and bad news for police forces trying to catch them."
Government Minister and Walsall North MP Eddie Hughes, said: "Once again the PCC is looking for a scapegoat to blame for his inability to do his duty in respect of holding the Chief Constable and the force to account.
"Any changes associated with Brexit will only be relevant in a small percentage of cases.
"The PCC is clearly winding down to retirement and the sooner he is replaced the better."