Phone tap evidence should not be used in court, the government's spying watchdog has said.
Phone tap evidence is presently not admissible in court
The Interception of Communications Commissioner Sir Swinton Thomas said doing so would damage the work of MI5, MI6 and the police.
The former Appeal Court Judge also said the number of mistakes made when applying for warrants allowing intercepts was "unacceptably high".
The government has been under pressure to allow such evidence in some cases.
'Errors and breaches'
Opposition parties and some civil liberties groups have called for phone tap evidence to be used - the UK is one of the few Western nations not to allow its admission in court.
But ministers have said that using phone tap evidence in public courts would put operatives at risk.
The security services, including government listening post GHCQ, are thought to oppose such a move because it could risk revealing their spying techniques.
Sir Swinton said in his annual report: "I am left in no doubt that the balance falls firmly against any change in the present law and that any amendment ... would, overall, be damaging to the work of the security, intelligence and law enforcement agencies.
"I continue, of course, to have an open mind on this subject if any major change should occur in the future."
The commissioner also warned that there were a "significant number of errors and breaches" of the rules in 2004.
Most of the mistakes involved agencies which have to apply for or approve the right to tap phones getting their target numbers wrong.
In one case, the Home Office which ratifies applications from other agencies, transposed some of the digits and the mistake was missed by its staff and MI5.
The report also showed that the home secretary had approved 1,849 warrants for the interception of telephones, emails and other mail.
Some 674 were still in force on 31 December 2004.