The alert will come into force at 12pm on Friday (June 27) and will remain in place until 6pm on Tuesday (July 1).
It was issued by the UK Health Security Agency, warning of a rise in deaths and pressure on services as the heatwave continues.
This is the second amber heat health alert issued this month, following the UK Health Security Agency issuing one on June 19.
London is among several regions covered by the warning, which also includes the East Midlands, East of England, the South East, and the South West.
The Met Office says London will see a top temperature of 27C on June 27, followed by 29C on June 28, rising to a peak of 31C on June 29, 34C on June 30, and reaching 31C again on July 1.
According to the alert, the high temperatures are expected to cause a rise in deaths, particularly among those aged 65 or over with health conditions.
Hospitals and care homes could become dangerously hot, putting vulnerable people who live alone at increased risk.
Staff may find it harder to deliver services, and problems with transport and power could make the situation worse.
The impact is also starting to reach beyond healthcare, with industries such as travel beginning to feel the strain.
Water safety is another growing concern, as more people head to rivers and lakes to cool off.
London Fire Brigade said it has already responded to more than 280 water-related incidents so far this year, a 15 per cent rise compared to the same period in 2024.
15 of those incidents resulted in drownings.
In light of this, the fire service is increasing its safety messaging as part of Drowning Prevention Week.
During a visit to Kingston Fire Station, Liberal Democrat leader Sir Ed Davey watched a live demonstration of the brigade’s river flume tank, a piece of equipment used to show how water flows and the hidden dangers it can pose.
Assistant commissioner Pam Oparaocha said: “With warm weather and school holidays on the horizon, we want people, especially young people, to enjoy our waterways safely, and that starts with education.
“Strong currents, underwater hazards and cold water shock can be deadly.
“It’s important to know the risks and to familiarise yourself with lifesaving equipment like throwlines, which are available along many of London’s waterways.”
Deputy Mayor of London for Planning, Regeneration and Skills Jules Pipe said: “London’s rivers, lakes and waterways can look very appealing when the temperatures rise; however, they can contain hidden dangers.”
He encouraged people to stick to safety guidance, especially with more hot weather expected as the city heads toward the summer holidays.
