One of the two men who murdered the Cooper twins has won a three-year reduction in the jail term he must serve before he is eligible for release.

Dennis Robinson will now be able to apply for parole after 12 years, rather than 15, after what appears to be a landmark judgement by the Court of Appeal yesterday. Appeal Court President Edward Zacca said Robinson's appeal against his life sentence for the 2005 double murder was allowed and the court recommended that he serve "at least 12 years before any application for his release".

The ruling is thought to be the first to contradict the Act of Parliament which dictates that convicted murderers must serve a minimum of 15 years behind bars before the Parole Board can consider them for release.

The Appeal panel of three judges said they would give the reasons for their decision in writing at a later date. Robinson, along with Kenneth Burgess, was jailed for life for the murders of 20-year-old Jahmal and Jahmil Cooper by the Chief Justice in 2006. Burgess bludgeoned the twins with a metal baseball bat at his Devonshire apartment in March 2005, while Robinson guarded the door.

He helped Burgess dispose of the bodies down Abbot's Cliff in Hamilton Parish. The pair failed earlier this year to get their convictions overturned.

Robinson's lawyer, John Perry QC, argued during this appeal that the 15-year minimum term was unconstitutional, because it was set by Parliament rather than the court.

"The deprivation of liberty by a body other than the court is instantly unconstitutional," he told a hearing earlier this month.

Crown Counsel Cindy Clarke told the court the mandatory 15-year prison term was not unconstitutional but she said a judge should be able to fix a sentence above that and the law needed to be reviewed.