The heat warning has been extended for the area due to the ongoing heat and humidity.
The Middlesex-London Health Unit says the warning will be extended through the weekend and until Sunday evening.
At that time, temperatures are expected to drop below 20 degrees Celcius.
A heat warning is issued when the forecast calls for a day-time high of 31 Celsius or higher with a minimum day-time low of 20 Celsius or higher for three consecutive days or longer.
A humidex of 40 or higher is expected for three consecutive days or longer.
The combination of high temperature and high humidity currently in London and Middlesex County can be very dangerous, the health unit says.
Anyone who finds they are suffering symptoms associated with extreme temperature conditions should seek medical attention immediately either by calling 911 or going to the nearest emergency department.
