LEGAL experts yesterday accused prison officials of misinterpreting the law and allowing prisoners out of jail early.

The dispute centres on cases where inmates are sentenced for a second offence before their first term is completed.

Sheriffs who want the prisoner's second sentence to be added on to the first have found Scottish Prison Service staff treat the two periods as concurrent and effectively ignore the new punishment.

Leading bar association figures say the service's interpretation is wrong, while the prison authorities insist they are correct. Politicians say the act at the heart of the row is confusing and needs reform.

One man serving an 11month sentence and due for release on December 31 last year was given an additional threemonth sentence on December 30. As a result, he expected to spend at least another six weeks in prison, but the prison service released him the next day.

Gerry Considine, of the Glasgow Bar Association, was surprised by his client's liberation because the sheriff had stipulated that the sentence should start from "today's date".

"I think it is very clear that if the sheriff says you have three months imposed as of today, then that is the sentence you will serve, " said Mr Considine.

"I know there is some confusion about how to interpret the act, but I don't see that SPS have any right to interfere with the sentence handed down by the sheriff."

Paul McBride, QC, vicechairman of the criminal bar association, said there had been several examples of the prison service "wrongly" interpreting the law on second sentences. He said: "The position is as clear as crystal. The sentence should start on the day it is imposed. In this case the man should have served another six weeks. It does not make any sense otherwise."

A spokesman for the prison service said each sentence was calculated by them according to the Crime and Disorder Act 1998 and the information on the court warrant.

"If the half-way point for someone serving an 11-month sentence was December 31 and they received an additional three-month sentence on December 30, that would be subsumed within the earlier sentence, because the half way point of the second sentence falls within the continuum of the first sentence.

"The warrant issued by the court says whether it is consecutive or concurrent. Some lawyers may say this is the wrong interpretation, but we would dispute that."

Kenny MacAskill, the SNP justice spokesman, said: "This is a mess. The law has to be understandable, not just for sheriffs but for members of the public. Prison sentences should not be lightly given but, when they are, the public has the right to expect that they will be served and that they will not simply fall by the wayside."

The dispute emerged as ministers faced criticism for their new bill designed to slash reoffending rates and stem the rise in the prison population.

The main elements of the Management of Offenders etc (Scotland) Bill are to be greater co-operation between the prison service and social work staff to help ease ex-prisoners back into society, and a new curfew system of early release and electronic tagging.

However, they were criticised for putting huge reliance on the curfews, which have been used in England for the last six years to little effect. The Home Office's own research found that the primary benefit was saving money, not reducing re-offending.