The criticisms, from human rights charity Amnesty International and child welfare charity The Children's Society, come as the government prepares for a Commons challenge to its Immigration and Asylum Bill.

In a report published on Monday, Amnesty said the number of unaccompanied children arriving in the UK rose from 631 in 1996 to 2,833 in 1998.

Refugees 'face poverty, ill-health and disrupted education'

The children were mainly from war-torn regions such as Kosovo, Somalia and Afghanistan, it said, and had often been put on a plane by friends or relatives after their parents were killed or imprisoned.

Many were sent with false documents claiming they were adults, and were not identified as children even when they arrived in the UK.

Amnesty said that at least 76 unaccompanied refugee children, some as young as 13 or 14, ended up being held in detention in adult prisons over an 18-month period.

The charity demanded that the bill should be amended to incorporate the full United Nations Convention on the Rights of the Child, and to give all unaccompanied children a guardian to help them sort out their legal status.

The Home Office rejected Amnesty claims that unaccompanied children were subject to inhumane treatment by UK immigration officials as "hysterical and untrue".

A spokesperson said: "The image depicted of British civil servants deliberately terrifying vulnerable young children is outrageous and unjustified."

Poverty and ill-health

The Children's Society report, supported by welfare charity the King's Fund, found that many refugee youngsters faced poverty, ill health and a disrupted education while in the UK. They also suffered high levels of discrimination and harassment.

Straw has made concessions to head off a rebellion

It found refugee families faced great difficulties accessing health services and sometimes had to rely on hospital accident and emergency wards.

The government will discover the size of the opposition to its Immigration and Asylum Bill when it returns for its final stage through the Commons on Tuesday.

There had been fears of a large-scale backbench rebellion over plans to discourage bogus asylum seekers by replacing benefits for basic needs with a voucher system and a small amount of cash.

Labour MPs had complained that under the original provisions of the bill, adult asylum seekers would receive just £7 a week in cash and child asylum seekers £3.50, plus vouchers.

But last week Home Secretary Jack Straw announced concessions which included increasing the amount of cash to £10 for adults and children.