THIRTY cases were dropped in the past three years because key prosecution witnesses lost their memories or contradicted themselves in court, Mr Mathews said.
But courts were powerless to require witnesses to prove they suffered from memory loss, although they could be jailed for staying silent.
Mr Mathews faced questions from James To Kun-sun after the acquittal of Emperor chairman Albert Yeung Sau-shing on charges of blackmail and criminal intimidation.
Five key witnesses claimed memory loss.
Legislator Conrad Lam Kui-shing asked: 'Have you tried to prove these types of memory loss?' Mr Mathews said the prosecution could not question witnesses' claims of memory loss. But a witness declared hostile could be cross-examined by the prosecution on earlier statements.
It was up to the courts to allow witnesses to refer to earlier statements to refresh their memories. Cases dropped because of insufficient evidence could not be reopened if witnesses regained their memories, he said.