TORONTO - A key plank in the federal Conservatives' law-and-order agenda is the target of a constitutional challenge, in which lawyers argue a new law punishes people who exercise their right to a presumption of innocence.

Legislation slashing the credit offenders get for time served in pre-sentence custody infringes charter rights -- disproportionately affecting black and aboriginal offenders -- and should be struck down, lawyers argue in documents filed with the Ontario Court of Justice.

The Truth in Sentencing Act, which came into effect in February, did away with two-for-one credit offenders were receiving for time served before their sentence. Instead, courts are now required to limit credit to one for one, or if circumstances justify it, up to a maximum of 1 1/2 days credit.

Two-for-one credit was generally applied to reflect that remand facilities are harsher environments than correctional facilities and lack educational and rehabilitation programs.

Taking away the double credit leaves a person in pre-sentence custody subject to greater punishment than someone who, for example, was out on bail before being convicted, lawyers argue in court documents filed in advance of the hearing.

At the heart of the challenge is Marvin Johnson, who pleaded guilty to cocaine trafficking on May 4. His lawyers launched the constitutional challenge, arguing the Criminal Code amendments infringe upon Johnson's right to liberty.

The constitutional arguments are set to be heard at Johnson's sentencing hearing -- scheduled for Friday, Monday and Nov. 17 -- though his lawyer doesn't expect them to be addressed as early as Friday.

The law imposes lengthier sentences on people who are held in pre-sentence custody and exercise their right to be presumed innocent until proven guilty at trial, Johnson's lawyers Corbin Cawkell and Kristin Bailey write.

"An accused detained in custody who exercises this right is also going to be penalized for doing so, as he or she will necessarily be subject to one-for-one credit for pre-sentence detention," they write.

"The disproportionately lengthier sentence that results by requiring the state to prove an individual guilty beyond a reasonable doubt is inconsistent with fundamental justice."

The Criminal Lawyers' Association is supporting Johnson's application.

"The accused who hires a lawyer, waits for disclosure and then waits for a trial is forced to sacrifice his liberty at the cost of exercising these rights," it says in court documents.

The legislation also unfairly affects black and aboriginal offenders, amounting to differential treatment based on race, the lawyers argue. Johnson is black and of aboriginal heritage.

"Aboriginal and black accused, who already are more likely to be refused bail and are more likely to be sentenced to longer prison terms, will be affected disproportionately if they are subject to a determination that their pre-sentence custody is capped," they say.

The court documents go on to further criticize the Truth in Sentencing Act, arguing there is no evidence of any cost savings, and that it actually increases court delays by leading to lengthier bail hearings and sentencing hearings.

The Crown argues in court documents that Johnson's arguments haven't demonstrated the law will result in "grossly disproportionate sentencing of offenders."

The law applies to all offenders equally, even those who are black or aboriginal, Crown lawyers write.

"While (the law) may disadvantage the applicant by limiting his credit for pre-sentence custody, that disadvantage has nothing to do with either prejudice or stereotyping," they argue.

However, the Crown notes that even if the court finds the law constitutionally invalid, it would be of no effect. Only the Superior Court can strike down the law.

The Ontario Court may be able to permit Johnson a greater credit than permitted by the legislation, but it would not be able to make a declaration about the legislation's validity.