A drug used to treat UK patients with malaria has failed for the first time, scientists have found.
Four patients who presented with signs of the tropical disease, which is spread by mosquitoes, had to seek alternative treatment after the drug commonly used to tackle malaria did not work.
Research by the London School of Hygiene and Tropical Medicine (LSHTM) said the treatment failure was due to strains of the disease showing reduced susceptibility and a "potential first sign of drug resistance" to artemether-lumefantrine (AL).
Dr Colin Sutherland, who led the study, said treating patients with AL in the UK is "considered to be working well" but that the strategy "might need reviewing".
He said: "Fortunately there are other effective drugs available.
"All the patients were identified by self-referral which suggests more cases of treatment failure in the UK may have occurred.
"Frontline doctors should be alert to the possibility of artemisinin-based drugs failing, and assist with the collection of detailed information about specific travel destinations.
"A concerted effort to monitor AL outcomes in UK malaria patients needs to be made.
"This will determine whether our front-line malaria treatment drug is under threat.
"The good news is that our collaborative infectious disease warning and testing systems which are in place, overseen by Public Health England (PHE), are highly effective."
According to the latest statistics from the World Health Organisation (WHO), nearly half of the planet's population is at risk of malaria.
In 2015, there were roughly 212 million malaria cases and an estimated 429,000 malaria-related deaths.