Facilities at four short-term immigrant holding centres have been condemned as "inadequate" by the prisons watchdog.
Ministers said they take detainees' welfare 'extremely seriously'
Dover Asylum Screening Centre, a centre at London City Airport and two at Gatwick Airport are not suitable for overnight stays, its report says.
Detainees were found to have slept on tables or plastic chairs, it adds.
The Home Office said it tried to minimise the time people were held but might agree to a report recommendation for independent monitoring in future.
There are 12 centres at UK ports and airports where immigrants and their families can be held on arrival to the country while their claims are investigated or before they are deported.
Immigrants are only supposed to be detained for a few hours, but Chief Inspector of Prisons Anne Owers said people were sometimes held overnight, and occasionally for up to 36 hours.
At Gatwick, in the last few months alone 10 detainees had stayed more than 24 hours, she told the BBC.
"We found detainees sleeping on tables or in plastic chairs, sometimes without adequate heating, blankets or bedding," her report says.
"None of the facilities we inspected was suitable for overnight stays."
She told the BBC this was the first independent inspection of the centres and called for regular monitoring.
While staff were "good" to the detainees, they had become too used to the inadequate facilities on offer and so needed an outside eye.
The people detained in such centres were a "huge variety", she continued.
"But they will all be quite uncertain about what is happening next and have need of proper facilities, proper support and the ability to contact friends and family outside."
The watchdog's inspection also found the centres, which are all privately run, were not subject to regular visits from healthcare staff and had not instituted a complaints procedure.
Ms Owers said none of the centres had adequate child protection arrangements.
And the centre at London City Airport was "completely unsuitable" for holding children, her report says.
But the Home Office said it did not consider children to be at risk of harm in any one of the facilities.
"We take the welfare of detainees extremely seriously and as such we recognise that there may be a need to put in place a system of independent monitoring of these short-term detention facilities," said Immigration Minister Tony McNulty.
"It is important to emphasise that these facilities are non-residential holding rooms and are intended to hold people very briefly - usually for no more than a few hours."
The report acknowledges detainees are "treated well and with respect by staff" and since the inspections took place work has been undertaken to improve conditions, he added.
A spokesman for GSL UK Limited, which was in charge of the centres at the time of the inspections, said it was inappropriate to comment as the company no longer ran them.
The centres have since been taken over by Group 4 Securicor.
The unannounced inspections of the facilities at Dover, London City Airport, and Gatwick Airport's north and south terminals took place between November 2004 and January 2005.