Hundreds of children are being detained at the UK's ports and airports, figures have shown.
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".
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.