A watchdog has called on the Home Office to examine why planned removals of detained immigrant families "routinely" fail.
Chief Inspector of Prisons Peter Clarke recommended the step as he revealed that nearly four in five groups of adults and children placed in one facility were ultimately released, rather than leaving the UK.
He also highlighted how the unit near Gatwick Airport was being used even less than the centre it replaced, which was shut by the Government on value-for-money grounds.
Mr Clarke said: "Detaining families is very costly and carries a considerable human impact, especially for children.
"The bar for detaining children with their families has rightly been set high.
"However, the routine failure of detention to achieve the objective of removal suggests that it could be set still higher, and careful consideration should be given to the purpose served by pre-departure accommodation."
Mr Clarke recommended that the Home Office analyse why so many removals fail as he published a report on pre-departure accommodation (PDA) at Tinsley House immigration removal centre (IRC) in West Sussex.
The PDA holds adults and children facing removal under the family returns process.
Detention at the unit, which can last no more than five days, is a last resort when other removal attempts have failed.
As of April, when the inspection took place, 19 families had been held in the pre-departure accommodation since it opened in June 2017. Only four were eventually removed.
"This was troubling given the harmful effect that arrest and detention inevitably has on children who witness their parents becoming very distressed," Mr Clarke said, adding that during the inspection, children saw their parents being physically restrained.
The report said: "The arrest, detention and attempted removal of families from the UK was harmful to children but was often ineffective."
Two arrests observed by inspectors involved a team of more than eight uniformed officers wearing stab vests and heavy boots.
Of the 19 families that had been detained, five were released following a legal challenge, the report said.
While the number of successful removals following detention remained "very low", the inspectorate noted that more than 1,300 families had been returned without being detained in around two years.
Mr Clarke pointed out that the two-apartment facility at Tinsley House was being used "even less frequently" than the Cedars accommodation centre it replaced.
The Government announced that Cedars, once described as "palatial", was to close in 2016 following criticism it was underused and too expensive.
Inspectors said the new unit was located in a "more restricted space" and could not fully replicate the "open" environment at Cedars.
Nevertheless, the Tinsley House accommodation was "well-designed", with "comfortable" apartments, a games and music room, and a wide range of entertainment facilities.
Mr Clarke said staff at the PDA provided an impressive level of care and support.
In a separate report, he also praised the main IRC, saying it was "calm and stable" with a "largely positive atmosphere".
The Home Office said the number of families entering and being removed via pre-departure accommodation were a small proportion of the total number returned through the family returns process.
A spokesman said: "Detention is an important part of the immigration system - but it must be fair, dignified and protect the most vulnerable.
"This report makes many positive findings, including the good relationship between staff and detainees, however it rightly points out areas where we can do better and we are committed to improving."
The department pointed to a series of reforms to immigration detention announced by Home Secretary Sajid Javid last month.
Sarah Newland, the head of Tinsley House IRC and pre-departure accommodation, which are managed by G4S, said: "The report shows that while we do have improvements to make around diversity monitoring and promotion, and extending the use of the cultural kitchen, we can be proud of the work that we are doing to ensure that the detainees at Tinsley House are well cared for and supported, prior to their resettlement."
She added: "I am delighted that inspectors were positive about the PDA ... and recognised the dedication of the staff team who work with very vulnerable families and children."
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