Senior appeal judges admitted yesterday they had made a blunder when giving a ruling and had left a prisoner serving too long behind bars. In a rare move, Mr Justice Simon Mayo suggested the convicted drug trafficker, Chan Yat-sing, seek to reopen the case because 'he may be able to demonstrate that a substantial and grave injustice has been done'. The error by the Court of Appeal in March set a precedent which was later acted on by a judge passing sentence in a different case. That case came up for appeal before the same three judges, Mr Justice Mayo, Mr Justice Noel Power and Mr Justice Michael Stuart-Moore yesterday. The mistake made by the court in Chan's appeal had been to fail to give him a one-third discount on his sentence due to his admission of guilt. Chan had been caught with 95 grams of 'ice' but insisted it was only for his own consumption and he had no intention of selling it to others. He had been willing to plead guilty to possessing the drugs. But the prosecution put him on trial for trafficking. When he challenged the five-year sentence imposed by Madam Justice Clare-Marie Beeson, the Court of Appeal set it at 3.5 years. But they failed to take into account the fact he had always been prepared to admit possessing the drugs and was entitled to a reduction on his sentence. When Madam Justice Beeson later came to sentence in a similar case, she applied the same principles as in the Court of Appeal ruling. She jailed Lau Chi-chiu, 27, for three years, for possessing 121 Valium tablets and 63 grams of 'ice'. Corinne Remedios, for Lau, argued that the appeal court had been wrong when dealing with Chan's appeal. Mr Justice Mayo said that now the previous error had been realised, Lau's sentence must be reduced to two years.