Allowing phone-tap evidence to be used in court would not help secure many convictions in terrorism cases, the government's intercept watchdog says.
The government is considering allowing phone-tap evidence
Intelligence agency studies had found few cases, if any, where it would have made a big difference, Sir Swinton Thomas told BBC Radio 4's File on 4.
He said changing the law could harm law enforcement and security services.
But Director of Public Prosecutions Ken Macdonald said intercept material could help build stronger cases.
Sir Swinton, the outgoing interception of communications commissioner, said MI5, MI6 and GCHQ had carried out extensive trials to check if intercept material would be valuable in prosecutions.
"Those inquiries have shown very clearly that the number of cases where intercept material would make a substantial difference are very few indeed or possibly even non-existent," he said.
The DPP and the attorney general both support a change in the law to allow intercepted phone calls, e-mails and letters to be used in prosecutions of people suspected of terrorism and organised crime.
But Sir Swinton said: "I would certainly disagree with them. I feel this very strongly because I think it is so enormously important from the point of view of protecting the citizens of this country from terrorism.
"I deeply believe that if there is a change in the law, huge advantages which the security services and law enforcement agencies have at present would be lost."
The retired appeal court judge added: "I have no doubt that the view that I have expressed is correct."
However Mr McDonald insisted that "if we get to use this material we will be able to build stronger cases against some of the most dangerous people that we're dealing with, including terrorists.
"All the evidence from abroad is that you get priceless material which can be deployed to great effect in court."
Attorney General Lord Goldsmith told BBC News in September that intercept evidence could be a "key tool" to convict dangerous criminals.
Although no explicit mention of a bill to accept intercept evidence was mentioned in the Queen's Speech, Lord Carlile, the government's independent reviewer of terrorism legislation, told File on 4 he expected to see such evidence used in court by the beginning of 2008.
A Home Office statement said: "There is ongoing work to review these issues and identify, if possible, a legal model which would provide the necessary safeguards to allow intercept material to be used as evidence.
"Officials will report by the end of the year."
Hear the full story on Radio 4: File on 4 Tue 21 Nov 2000 GMT or online at the File on 4 website