Legal affairs analyst
As the law stands, the previous convictions of a defendant are revealed only at time of sentencing but not during the trial.
The prime minister wants to stop people "playing the system".
Where judges do allow jurors to know of a previous conviction, it will be accompanied by a strict health warning which urges them to take the information into account only to assess the defendant's credibility and not as a guide to guilt or innocence.
There is already a rule which allows "similar fact" evidence to be adduced by the prosecution.
For example, if an alleged rapist is accused of tying his victim's hands with rope and has been convicted of doing so previously, the prosecution might be able to introduce that in evidence.
But often, judges are not persuaded that such a course would not prejudice a fair trial.
'Guilt by association'
In 2001, the Law Commission recommended that these rules were too complex and restrictive. It suggested changes which would preserve the existing safeguards for the defence.
Critics say the government has gone much further with its proposals, opening the door to "guilt by association".
Law lecturer John Hodgson points out that the Law Commission set the bar, permitting the introduction of evidence of a defendant's bad character, at a high level, whereas the government is broadly placing the onus on the defence to exclude such evidence.
"It will inevitably lead to lazy convictions, based essentially on copious 'previous' and the likely prejudice this will engender in the jury or bench," he said.
It is no surprise that the two categories of offence for which the new rules will be brought in are theft and sex crimes against children.
The former is an area with a large number of repeat offenders and the latter probably causes the most alarm and frustration amongst the public.
The Law Society has warned that courts will be confused because the government has brought forward the introduction of the changes by several months without adequate time for training of judges and magistrates.
And civil liberties groups have wondered whether the proposals might breach the right to a fair trial under the European Human Rights Convention.
The government counters by pointing out that judges will retain discretion to exclude evidence where they conclude that the prejudicial effect on the jury would outweigh any pointer towards guilt.
The traditional independence of the judiciary might give this protection considerable teeth. Perhaps more than ministers would like.