Jaap Doek, chairman of the UN committee on the rights of the child, called for urgent action to stop abuses, recalling the death of two children in custody this year and asking why Britain tolerated the unnecessary jailing of juveniles.

He commended a report today from the Children's Rights Alliance for England drawing attention to slow progress in tackling youth poverty. It criticises teenage workers' low wages, the withdrawal of rights from child asylum seekers, and a discriminatory system that makes it three times more likely that young black people will be in prison than at university.

The attack came as Gordon Brown, the chancellor, was preparing to make increased spending on children a centrepiece of his pre-Budget report on Thursday. Margaret Hodge, minister for children, is due to publish the government's childcare strategy the same day.

The committee on the rights of the child inspected UK performance in 2002 and made 78 recommendations for improvement, including calls for a ban on smacking and reform of juvenile justice. The alliance says the government has so far made progress on only 17.

Mr Doek said: "The UK will next be examined by my committee in 2009. That is too long to wait for children whose human rights are being violated today. Urgent action is required to remedy the plight of children in custody ... many children are officially classed as too vulnerable for prison service custody, and there are continuing and grave concerns about children's access to education, health care and protection.