It pressed Sir Patrick Mayhew, the former Attorney General, for permission to bring the men to trial after lawyers warned it had a 'less than 50 per cent chance of success'.
Sir Brian Unwin, chairman of Customs, Sandy Russell, his deputy, and Customs solicitor Michael Saunders, argued that 'exceptional circumstances' justified the case coming to court.
Mr Russell warned that failure to prosecute after parts for the Iraqi supergun were seized at Teesport docks, in April 1990, would deliver a 'hammer blow' to Customs' credibility.
Peter Mitchell, managing director of a company which helped to manufacture the gun parts, and Christopher Cowley, a metallurgist, were both charged with breaches of export legislation.
Sir Patrick decided the case was not exceptional and there was inadequate evidence for a prosecution, the inquiry was told.