A man's death at the Manston migrant processing centre may have been caused by a diphtheria infection, the Home Office has said.
Initial tests at a hospital near the centre in Kent, which has struggled with overcrowding and outbreaks of disease, came back negative - but a follow-up PCR was positive.
The result indicates that "diphtheria may be the cause of the illness", a statement said.
The man died in hospital on November 19 after being believed to have entered the UK on a small boat seven days earlier.
A Government spokeswoman said: "Our thoughts remain with the family of the man who has died and all those affected by this loss.
"Initial test results processed by a local hospital for an infectious disease were negative, but a follow-up PCR test was positive, indicating that diphtheria may be the cause of the illness. The Coroner will conclude in due course.
"We take the safety and welfare of those in our care extremely seriously and are taking all of the necessary steps following these results.
"We are offering diphtheria vaccinations to people at Manston, which has 24/7 health facilities and trained medical staff."
A post-mortem examination and a coroner's investigation are ongoing.
It comes as it was claimed migrants who still had diphtheria have been transported to hotels around the country as ministers emptied the Manston centre this week, risking a wider outbreak of the disease.
The Sunday Times said there have been 70 cases of diphtheria among those who have been moved from the facility, including 27 men who were diagnosed at a hospital in Wakefield, prompting an "incident" to be declared locally.
Diphtheria is a highly contagious infection affecting the nose, throat and sometimes skin. The NHS says it is rare in the UK and can be treated through antibiotics and other medicines.
The Home Office initially said there was "no evidence at this stage to suggest that this tragic death was caused by an infectious disease", with the man being taken ill a day before dying.
New arrivals were expected to be taken to the site, which is designed for holding people for short periods during security and identity checks before they are moved to accommodation.
But some people have been held for far longer periods due to a lack of alternative accommodation.
The Home Office has not said whether it is taking steps to limit the spread of diphtheria elsewhere, with migrants having been moved from Manston to hotels around the country.
Ahead of the man's death, the UK Health Security Agency identified 39 diphtheria cases in asylum seekers in England in 2022, as of November 10.
The health authority warned that accommodation settings should be considered "high-risk for infectious diseases".
Home Secretary Suella Braverman has been coming under criticism for the dire conditions in Manston, and for failing to slow the perilous crossings of the English Channel by people in small boats.
This week she admitted the Government has "failed to control our borders" but blamed desperate migrants and people smugglers for the overcrowding in Manston.
"I tell you who's at fault. It's very clear who's at fault. It's the people who are breaking our rules, coming here illegally, exploiting vulnerable people and trying to reduce the generosity of the British people. That's who's at fault," she told MPs.
Liberal Democrat MPs called for the immediate resignation of Ms Braverman.