California attorney general Bill Lockyer has dropped criminal charges against former HP chairman Patricia Dunn following the company's boardroom spying scandal.

"We have maintained from the beginning that Patricia Dunn was innocent and thus vigorously fought the charges against her," said James Brosnahan, lead counsel for Dunn at law firm Morrison & Foerster.

"Today, the judge dismissed the case. Ms Dunn did not plead to anything. This is the right result."

A former HP lawyer and two private investigators pleaded 'no contest' to misdemeanour charges.

The court did not accept their plea deal, but offered to dismiss the charges if they completed 96 hours of community service and paid restitution.

A third investigator charged in the case earlier this year agreed to act as a witness for the prosecution.

HP fraudulently obtained phone records and personal information from reporters and board members in 2006 as the company attempted to trace the source of an information leak from its board of directors.

Dunn oversaw the investigation, but has always maintained that she had no knowledge that investigators used illegal methods.

The scandal prompted the firing of Dunn, who was replaced by the company's chief executive Mark Hurd in September.

Lockyer filed criminal charges against Dunn in October. The other four were charged for fraudulently obtaining phone records, unlawfully accessing and using computer data, identity theft and conspiracy.

The charges carried a combined maximum prison sentence of 12 years and a $55,000 fine.