Health experts have issued a warning about malaria, after four people from north-west England contracted the disease during holidays overseas.
Malaria is spread by mosquitoes
Liverpool School of Tropical Medicine said they caught a deadly form of malaria in Gambia, West Africa, after travelling without proper protection.
Two of the victims remain seriously ill in hospital.
Dr David Lalloo, the school's clinical director, said unnecessary deaths from malaria continue to rise among Britons.
He said thousands of UK travellers were putting their lives at risk by ignoring the threat of malaria when they go on exotic holidays.
The Health Protection Agency (HPA) said that two people from the UK had died recently from malaria and that they had not taken the appropriate anti-malarial medication.
A spokesman for the HPA said the locations of the victims were not being released.
The Liverpool School of Tropical Medicine said two Merseyside men fell ill with the mosquito-borne disease following a fishing trip four weeks ago.
One has since been discharged from a Liverpool hospital, but the other is still very ill in intensive care.
A Merseyside woman who went on holiday to Gambia in November also returned with malaria, but has now been discharged from the same hospital.
A fourth tourist remains very ill in another North West hospital.
"As we understand it, none of these four tourists had taken adequate protection," said Dr Lalloo.
"This underlines the importance of seeking medical advice before travelling to malaria-endemic countries."
He said the number of people returning to Britain with the potentially fatal form of malaria has grown from about 250 cases in 1977 to up to 1,500 annually in recent years.
Malaria is an infectious disease carried by mosquitoes which pass on a parasite into their victim's bloodstream.