Criminals frequently receive unduly lenient sentences because judges struggle to understand the complex and antiquated laws on sentencing, the law commission has said.

The commission warns public confidence in criminal justice risks being harmed when “sentencing decisions are routinely unlawful, unduly lenient or otherwise inappropriate because of the incomprehensible nature of the current law.”

It says judges are having to contend with more than 1,300 pages of law full of outdated, inaccessible language from more than 65 different Acts of parliament, some more than six centuries old.

It proposes to strip it down to a single simplified sentencing code, enshrined in a new law, which it says would reduce mistakes and save £250m over 10 years by cutting the time it takes for judges to come to decisions and the number of appeals.