Hundreds of children are being detained at the UK's ports and airports, figures showed today.
A total of 697 children under 18 were held for up to 24 hours at the Port of Dover and at airports including Heathrow, Gatwick and Stansted in the four months between May and August.
The Children's Society, which obtained the details under the Freedom of Information Act, said the figures raised serious questions about the Government's commitment to end the immigration detention of children.
Of those children detained, one in three was unaccompanied, the figures showed.
Bob Reitemeier, the charity's chief executive, said: "We are horrified at the excessive numbers of children being held in the South East and very disappointed that Government has not kept these numbers to a minimum.
"It is of great concern that this appears to be happening without sufficient monitoring centrally by the Home Office, including why they are being held, their age and critically the length of time that they were held.
"This raises serious questions about the commitment to end the immigration detention of children."
He urged the Home Office to launch an investigation into "why excessive numbers of children are being held on entry to the UK".
The Home Office should also "make sure appropriate measures are in place to meet the welfare needs of what are often extremely vulnerable children", he added.
A UK Border Agency (UKBA) spokesman said: "We have always been clear that we would retain the ability to hold families who have arrived at the border without the right to enter the UK.
"Where it is considered in the family's best interests not to stay at the airport until the next flight, the UK Border Agency will make arrangements for them to stay at Tinsley House."
A report in April by the independent monitoring board (IMB) at Heathrow Airport warned that children were still being held overnight in "degrading" and "wholly unsuitable" conditions.
They were among more than 15,000 people who were detained by immigration officials at Britain's biggest airport last year in rooms with no natural light, poor ventilation and inadequate washing facilities.
The lack of progress since the "degrading" conditions were highlighted last year was "unacceptable on grounds of humanity", the watchdog said.
"The UK Border Agency has again failed in its duty to treat everyone in its care in Heathrow holding rooms with decency," the report said.
"In our last report we drew attention to the wholly unsuitable conditions in which men, women and children were held.
"There has been no change: they are still held in these conditions and still for too long. Lack of change is unacceptable on grounds of humanity.
"We are opposed to the continued detention of families for immigration purposes at Heathrow."
It called for Home Secretary Theresa May to review urgently the UKBA's powers for improving the accommodation and to give priority to the provision of a short-term holding facility offering overnight accommodation at, or near, the airport.
David Wood, the UKBA's strategic director for criminality and detention, said at the time that the welfare of children was "an absolute priority".
But he added that families with children may be held on arrival in the UK "while checks are made to determine whether they should be admitted to the country and, if not, until a return flight can be arranged for them".
Last month, ministers welcomed the "final stage in the Government's pledge to end the detention of children" as new pre-departure accommodation for families being removed from the UK was opened.
Barnardo's provides help and support to the families while they are being held in the new Cedars centre in Pease Pottage, West Sussex, for up to a week before being removed.
The children's charity has also laid down a series of rules for the Government to follow to ensure it keeps the charity's support in removing children of failed asylum seekers from the UK.
But critics have accused Barnardo's of "legitimising detention".