A person staying at the Manston migrant processing centre in Kent has died in hospital, the Home Office has said.
They were admitted after becoming unwell and died on Saturday morning.
It is understood the person travelled to the UK by small boat and arrived last Saturday.
There is “no evidence at this stage” to suggest the death was “caused by an infectious disease”, the Home Office said.
There will be no detailed comment until a post-mortem examination has been carried out, the spokesman 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,” he said.
The Home Office tweeted that it was “profoundly saddened by this event” and sent its “heartfelt condolences to all those affected”.
Manston has been at the centre of controversy in recent weeks.
It is designed to hold up to 1,600 people for no more than 24 hours but, earlier this month, there were about 4,000 on site.
Temporary marquees were set up to house the extra people.
Some migrants were threatening to self-harm and go on hunger strike, with unrest “spreading across the camp”, Sky News was told.
A farmer from Eritrea said he slept on cardboard and was given cold hot dogs for lunch.
Others begged for help via a message in a bottle thrown over the perimeter fence.
Earlier this week it was confirmed that more than 40,000 migrants had crossed the Channel this year.
Suella Braverman, the home secretary, was criticised after telling MPs there is an “invasion on our southern coast”.
The UK has agreed a deal with France to try to reduce the number of people making the dangerous journey.
But migration and policing specialists said £8m in extra funding, a 40% increase in officers on French beaches, and enhanced intelligence sharing would not be enough to bring numbers down.