Juries in child sex abuse and theft cases in England and Wales could soon be told whether the defendant has convictions for similar offences.
David Blunkett wants victims put at "the heart" of the justice system
Prime Minister Tony Blair and Home Secretary David Blunkett said the move was designed to put victims first.
An order detailing the plans, which include widening the offences covered, was laid in Parliament on Monday.
Human rights campaigners say the changes are dangerous and "guaranteed" to cause miscarriages of justice.
At present, juries are rarely told if the accused has previous convictions for fear of hindering a fair trial.
The proposal will have to be approved by both Houses of parliament.
Cases where the law could have been applied in the past include the trial of Roy Whiting, who abducted and murdered eight-year-old schoolgirl Sarah Payne in July 2000.
The killer had a previous conviction for the abduction and indecent assault of a nine-year-old girl in June 1995.
But the jury in the Sarah Payne trial did not hear the details until they had reached a verdict.
Mr Blunkett said the reforms put victims "at the heart of the justice system".
'Search for truth'
He said: "Trials should be a search for the truth and juries should be trusted with all the relevant evidence available to help them to reach proper and fair decisions.
"The law has recognised for over a century that evidence of a defendant's previous convictions and other misconduct may be admitted in some circumstances.
"But the current rules are confusing and difficult to apply, and can mean that evidence of previous misconduct that seems clearly relevant is still excluded from court."
Mr Blair said the change would help protect the rights of victims while convicting the guilty.
He said: "It's designed to make it clear that we're not going to have people playing the system and getting away with criminal offences that cause real misery."
Under the Criminal Justice Act 2003, judges will have discretion to allow bad character evidence - previous convictions - to be revealed from mid-December.
In most cases, details of convictions must be "strikingly similar" to the new allegations.
But Monday's order means that in child sex abuse and theft trials, the level of similarity between past offences and current charges can be lower.
The Home Office says the onus will be on making the information known.
'Miscarriages of justice'
Barry Hugill, spokesman for human rights group Liberty, said: "Most jurors would find it very difficult not to be influenced by admission of previous convictions.
"That means you would be trying someone not for their alleged crime but for the those previous crimes.
"It's guaranteed that it will lead to miscarriages of justice."
A Bar Council spokesman said: "We need to avoid a 'round up the usual suspects' culture, where those who have previous convictions could be increasingly presented to the courts as the sort of person likely to
have committed a crime."
The Law Society's Russell Wallman said: " The Government had planned to implement these changes in spring 2005.
"There is a real danger that we will see inconsistent and erratic approach.
"This rushed approach risks damaging the credibility of the new rules."