Mr Hadfield has written to Mr Mullin following his suggestion that the West Midlands Police were 'the most corrupt police force in the country' and had wasted public money attempting to reconvict the Six.
His attack came in the wake of confirmation that the Crown Prosecution Service is considering a report on scientific tests carried out for the force's re-investigation of the 1974 pub bombings by the IRA.
The tests, using new methods on original evidential material, are suggested to shed important new light on the affair and could become an issue in the forthcoming trial of three detectives for perjury and conspiracy over interviews with one of the Birmingham Six, released by the Court of Appeal last year.
The letter from Mr Hadfield says: 'What appals me is the fact that you will stoop to suggesting corruption on the part of 7,000 police officers. Some of those officers were schoolchildren in 1974; during last year those same men and women investigated 300,000 crimes; they arrested 130,000 people; responded in a professional manner to almost 1 million calls for help; they are being assaulted almost daily, yet you brand them all as corrupt.
'Fortunately, police officers are resilient. They shake their heads in disbelief when, occasionally, they are castigated for trying to do their job.'