In a webchat on the Guardian website, the former undercover police officer who made allegations of a police plot to smear Stephen Lawrence's family has said a public inquiry could encourage more undercover officers to come forward.
Peter Francis said: "I personally think you will have more whistleblowers come out, but only if there is a proper public inquiry called. That is why I believe there is an overriding reluctance to call one."
Mr Francis went on: "That is the only thing that will bring out what the SDS has been doing since 1968 - but also the National Public Order Intelligence Unit, which still operates."
He said he will not provide evidence for either the Lawrence corruption inquiry or the police investigation into undercover officers.
"I will not cooperate at all with the two inquiries the Home Secretary said can deal with these matters, even if they are 'supervised' by the Independent Police Complaints Commission. I have no faith in any of these inquiries.
"Only a judicial-led or public inquiry - not just into the Stephen Lawrence allegations, but into the wider controversy - has any chance of ever establishing the truth.
"If there is a public inquiry, I will happily give all my evidence under oath, explaining what I personally know about the SDS and covert policing of protest groups."