Dr. Adrian Smith and colleagues of the Health Protection Agency's Malaria Reference Laboratory used data from the Malaria Reference Laboratory involving 39,300 confirmed cases of malaria.

Smith said 64.5 percent of 20,488 malaria cases were among travelers from Britain who had visited friends and relatives in malaria endemic countries. The number of British residents traveling to malaria endemic areas rose from 593,000 in 1987 to 2.6 million in 2004.

Imported malaria cases were heavily concentrated in communities that experienced frequent travel to see friends and relatives in West Africa, Smith said.

The researchers said there is a worrisome trend in cases of the potentially fatal falciparum malaria which have increased steadily during the past 20 years in the United Kingdom.

Of all malaria imported to the United Kingdom, 96 percent of falciparum malaria occurred after travel to Africa. Travelers to Nigeria and Ghana, neither common tourist destinations, account for half of all imported falciparum cases, the study said.

The findings are published in the British Medical Journal.