Health experts have renewed calls for people travelling to malaria-affected areas to ensure they take medication.
Malaria is spread by mosquitoes
The warning from the Health Protection Agency comes after three more people from Britain were diagnosed with malaria.
There have been a total of nine cases since mid-November. Two people returning from Gambia died.
None of the nine had followed medical advice to take anti-malarial medication, the HPA said.
It is particularly concerned about people going to Gambia in west Africa.
The HPA said people must take the tablets prior to travelling, while abroad and for a period after returning in order to be protected against malaria.
The number of people returning to Britain with falciparum malaria, the most severe form of the disease, 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.
Professor Peter Chiodini, a malaria expert from the HPA, said: "We are strongly urging British travellers heading to malarious destinations, such as The Gambia, to seek pre-travel medical advice and to take the anti-malarial medication that they should be prescribed prior to, during and after their trip.
"We are issuing this further warning following recent reports of another three cases of falciparum malaria in returning travellers."
He added: "Malaria is a preventable disease but since November we have been made aware of nine cases, including two fatalities, in returning travellers.
"We ask that travellers also avoid mosquito bites by wearing insect repellent and cover-up clothing and sleep under an insecticide-treated net in conjunction with taking anti-malarial medication to ensuring they adequately protect themselves against the disease."