Support groups have called for "urgent action" to overhaul the immigration system following the death of a man who was staying at the controversy-hit Manston migrant processing centre.
The Home Office has confirmed that a person, understood to be male, died in hospital on Saturday morning after "becoming unwell".
It is understood that he arrived in the UK as part of a small boat crossing on November 12. He is believed to have been taken ill on Friday evening.
Efforts are under way to inform the man's next of kin, it has been reported.
Now a support group founded by a group of volunteers with the aim of supporting the people of the Calais refugee camps is demanding immediate steps be taken.
Care4Calais founder Clare Moseley said: "Our condolences are with the family and friends of this man.
"Refugees we are supporting who have recently been released from Manston have told us they were denied access to doctors and medication.
"We continue to have concerns about the health facilities at the centre."
Freedom From Torture, which provides psychological support to asylum seekers, called for a "compassionate asylum system".
It posted on Twitter : "We've said it before: this Government's cruelty to refugees isn't accidental - it's the whole point.
"We need a compassionate asylum system that works. NOW."
Inquest, which supports bereaved families following a death in state care, has called for an independent investigation into the death and an overhaul of the system.
It posted: "It feels as if it was only a matter of time before a death like this happened in this completely closed facility.
"We need urgent action on Manston, independent investigation of this death & an overhaul of this inhumane immigration system."
The small boats crisis has dominated headlines over the past few weeks, with ministers under fire for overcrowding chaos at the holding centre in Kent.
A statement posted on Twitter by the Home Office on Saturday said: "We express our heartfelt condolences to all those affected."
"We take the safety of those in our care extremely seriously and are profoundly saddened by this event.
"A post-mortem examination will take place so it would not be appropriate to comment further at this time," the statement said.
A post-mortem examination is due to be carried out and the Independent Office for Police Conduct (IOPC) has been notified.
The Home Office said there was "no evidence at this stage to suggest that this tragic death was caused by an infectious disease".
It added: "We take the safety and welfare of those in our care extremely seriously and provide 24/7 health facilities with trained medical staff at Manston."
At one point as many as 4,000 people were being detained at the site, which is designed to hold just 1,600. This number later dropped to within capacity.
Asylum seekers are meant to be at Manston for only short periods of time while undergoing security and identity checks, before being moved to the Home Office's asylum accommodation.
Some people have been held for longer periods due to a lack of alternative accommodation, with concerns raised over poor conditions.
Shadow home secretary Yvette Cooper said there will need to be a "full investigation" into the incident.
The Labour MP said: "We send deepest condolences to the family of the man who has died after staying at Manston.
"There will need to be a full investigation into what has happened in this tragic case."
Enver Solomon, chief executive of the Refugee Council, said: "Every person in Manston must be looked after with the care and attention they need, so when a tragic death likes this takes place it is always a matter of serious concern.
"It is vital that a thorough and speedy investigation takes place to understand what happened and whether all the necessary procedures were followed."
A Home Office spokeswoman said no further details such as the age or nationality of the deceased would be released at the moment.
She also declined to comment on how the Home Office was responding to the circumstances to the death beyond the referral to the IOPC.
