A key malaria treatment has failed for the first time in patients being treated in the UK, doctors say.
The drug combination was unable to cure four patients, who had all visited Africa, in early signs the parasite is evolving resistance.
A team at the London School of Hygiene and Tropical Medicine said it was too early to panic.
But it warned things could suddenly get worse and demanded an urgent appraisal of drug-resistance levels in Africa.
Malaria parasites are spread by bites from infected mosquitoes.