The new chair of the Youth Justice Board expressed his fears over the use of custody as he revealed last week that the juvenile prison population has risen since Christmas.

Rod Morgan told the annual conference of rehabilitation agency Nacro that over the past 18 months the board had enjoyed moderate success in reducing the detention of children and thought it was "bucking the trend".

But, although the board expected the usual seasonal fluctuation after Christmas, the figure did not flatten out in March and was continuing to rise, he said.

"We aim to have 90 per cent occupancy rate in our secure places and 10 per cent spare capacity," he told delegates. "But today the capacity was 99.5 per cent full."

Morgan attributed the rise to the number of remand prisoners, which had "gone up steeply", and to children detained for breaching orders, including antisocial behaviour orders.

Morgan warned that the death of Joseph Scholes, whose inquest opened last week, was a reminder of the consequences of having a high prison population. Scholes, 16, was found hanging in Stoke Heath Young Offenders Institution in Shropshire in March 2002.

Morgan warned that, when the numbers rose, the pressures were greater and the risks increased.

But he welcomed the fact that the health care centre where Joseph died was being converted into a resettlement centre and new health care facilities were being built.

Morgan said the board wanted to give youth offending team managers more money to create and develop alternatives to custody, but he warned that this could only be achieved if the prison population was reduced.

"We need to persuade sentencers that there are credible, humane alternatives to custody," he said. "If numbers rise, we will face further problems."