The UK Border Agency is detaining children in degrading and disgraceful conditions at Heathrow Airport, it was claimed.
Children of all ages are held there for immigration purposes almost every day and sometimes spend the night there, says the Independent Monitoring Board (IMB) for the airport in its annual report.
They are held in rooms which are small, stuffy and have no natural light, the report says.
There is no access to the open air, no sleeping accommodation and only hand basins for washing. Children often share space with unrelated adults and can be there for many hours.
A spokesman said: "This continues despite the government saying that it would end the detention of children for immigration purposes.
"The IMB recognises that children cannot always be admitted to the country straight away and are sometimes held for their own protection.
"But it recommends that non-custodial, child-friendly accommodation is provided at Heathrow for families with children as a matter of urgency."
Other findings include that almost 3,000 people were held for more than 12 hours in 2011 and this was more likely to happen at Terminals 3 and 4, which have the worst accommodation, and that people being removed from the country often face long overnight journeys to Heathrow, rather than spending the night at one of the nearby Immigration Removal Centres.
The IMB is appointed by the Home Secretary to monitor and report on the welfare of people in immigration custody at the airport. Board members are unpaid volunteers, who visit the airport at least once every week.
The report says that it is now five years since the IMB for the non-residential, short-term holding facilities at Heathrow was established.
"During that time there has been welcome improvement in the care of detainees. The holding rooms are better equipped and there is a wider range of food. Particularly poor accommodation at Queen's
Building and Terminal 2 has closed. Above all, the detention custody officers (DCO) are much more attentive to the welfare of detainees in their care.
"However, the accommodation remains unsuitable for anything but a very brief period of detention and is quite inappropriate for holding children or for overnight use. Many detainees are held for long periods, including through the night. Children are detained almost every day and sometimes overnight. Improvements to the accommodation were promised in 2011 by the UK Border Agency, but very little has happened."
The report adds that Heathrow is the UK's busiest international port, where some 15,000 people are detained for immigration purposes each year.
"The conditions under which children are held and that detainees have to endure overnight are degrading and disgraceful," it says.
A Border Force spokesman said: "We share the Independent Monitoring Board's concerns about the quality of the accommodation provided by BAA. We have raised this with them on numerous occasions in the past and will continue to do so to ensure those held at the border have proper facilities to meet their and our needs.
"The report rightly recognises that we are handling cases efficiently and professionally and that it identifies improvements in the way passengers, including children, are treated by staff.
"We will respond to the report fully in due course."
A spokeswoman for Heathrow Airport's operator BAA said: "We have not seen the report and therefore can't comment but are somewhat surprised by UK Border Force's response, since we have had many meetings with them recently and it has not been raised."