Three mosquitoes in traps in three different parts of London have tested positive for West Nile Virus, the Middlesex-London Health Unit says.
The mosquitoes were found in traps in Pond Mills, east London and northeast London.
The health unit says Londoners shouldn't drop their guard against mosquitoes, despite the sunny weather.
"Finding mosquitoes that are carrying West Nile Virus isn't surprising, in fact this is about the time we expect to start finding adult mosquitoes that test positive," said Steve Tomé, a vector-borne disease field technician with the health unit.
"While we can trap and test adult mosquitoes and treat places vector mosquito larvae are found, we rely on local residents to help us reduce and repel these insects."
People should wear insect repellant that contains DEET, wear light-coloured clothing and make sure their properties are clear of standing water, where mosquitoes breed.
The majority of people who become infected with West Nile don't get sick, the health unit said.
The Middlesex-London Health Unit monitors about 260 standing water sites on public property for West Nile, and applies a biological larvicide where vector mosquito larvae is found.