The Home Office has been told to "get a grip" over the issues at the Manston migrant processing centre - as a new report reveals detainees were not allowed to close toilet doors fully and had to sleep on the floor.
Some migrants have not been allowed access to mobile phones to inform their families if they were safe, while other "exhausted detainees" have waited more than 30 hours to be processed, the report from HM Inspectorate of Prisons found.
Manston is a non-residential facility that holds detainees for a maximum of 24 hours while their initial immigration paperwork is processed.
The site is only designed to hold 1,000 people but there are currently around 4,000 migrants there - more than any UK prison population.
While there is access to food, water, showers and toilets, there are no beds and there is no access to fresh air or exercise.
Home Secretary Suella Braverman has faced criticism for the facility's overcrowding and has been accused of ignoring legal advice that said migrants should be sent to hotels from Manston.
But on Monday she said she "never ignored legal advice" and since she was appointed by Liz Truss in early September has "worked hard to find accommodation to relieve pressure at Manston".
However, she said illegal migration is "out of control" citing an "invasion on our south coast".
She also said the sheer numbers arriving via the Channel is making it impossible to provide accommodation for them.
Following the report's publication, Chief Inspector of Prisons Charlie Taylor has urged the government department and its contractors to speed up the processing of migrants and make "suitable provisions" so people can be moved off the site near Ramsgate in Kent as quickly as possible.
The inspection, which was carried out at the facility in July, warns that serious challenges remained for migrants crossing the Channel and arriving in Kent.
"The Home Office and contractors need to get a grip, they need to speed up the processing of migrants, they need to make suitable provisions so people can be moved off-site as quickly as possible and housed in humane and decent conditions," he told BBC Radio 4's Today programme.
Migrants are taken to sites at Western Jet Foil in Dover and Lydd Airport in Romney Marsh for health checks when they first arrive in Kent after crossing the Channel from Calais.
The Lydd Airport site was unoccupied when the inspection took place.
It reported "failings" in procedures at Manston that "undermine the resilience of the centre for dealing with increasing volumes of detainees".
However, it found the accommodation was suitable for short-term detention and noted efforts by staff to "create a calm and even welcoming atmosphere".
While Manston was found to have a "good amount" of accommodation available, at the time of the inspection, "much of it was out of use because there were not enough staff".
Other "signs of strain" included "exhausted detainees" sleeping on the floor, including some who had been waiting more than 30 hours to be processed.
It comes as Conservative backbencher Sir Roger Gale told Sky News overcrowding at the facility - where outbreaks of MRSA and diphtheria have been reported - is "wholly unacceptable".
The prisons watchdog also found:
• Victims of trafficking and vulnerable detainees such as those with disabilities were "not always assessed or recorded appropriately", with some not identified as "adults at risk"
• Some detainees were banned from using mobile phones to let their families know they were safe and, in parts of the site, others were "inexplicably" not allowed to close toilet doors fully
• Translation services were not always used so that detainees understood the situation.
Mr Taylor said the inspection, which included two other migrant processing centres on the south coast, unveiled a number of "risks" about the facilities.
He added that recent information from "a number of credible sources", including other watchdogs, suggested the current situation had "significantly deteriorated".
In a bid to combat the problems, he plans a "swift return" to Manston for another inspection when he would "expect to see substantial improvements".
"In the meantime, the Home Office and its contractors need to get a grip and urgently act on the findings of this report to make sure all detainees are held in safe, decent and humane conditions," he said.
A Home Office spokesman said: "We welcome the report's finding that there have been considerable improvements to the infrastructure and processes in place to accommodate record numbers of people arriving in the UK illegally via small boats.
"As a result of these numbers, our asylum system has been put under incredible strain, but we recognise there is more to do to provide alternative accommodation for people arriving in the UK. We continue to work hard to resolve the current pressures at Manston as an urgent priority.
"Manston remains resourced and equipped to process migrants securely, and we will provide alternative accommodation as soon as possible."