Britain could bake under record-breaking heat as temperatures could potentially climb to 39C.
The scorching conditions may spark thunderstorms which could trigger travel delays, flash flooding, and power cuts, the Met Office said.
The dangers of cooling off in lakes, rivers and the sea were highlighted by emergency services after the bodies of three people were pulled from the water after they reportedly got into difficulty swimming.
Searches continue for someone seen in the River Thames near Waterloo, Scotland Yard said.
A yellow weather warning is in force for most of England, except the south west, and parts of Scotland from 3pm on Thursday until 4am on Friday.
Overnight lightning storms have already caused fires and rail disruption this week during the heatwave.
According to the Met Office, there is a 70% chance the mercury could rise above the current all-time UK temperature record of 38.5C on Thursday.
Conditions could reach 39C in southern and eastern England, it tweeted.
Network Rail warned speed restrictions may be introduced in areas where tracks are at risk of buckling.
Extreme weather action teams (EWATs) have been "activated" to keep passengers safe and trains running, it said.
The Rail Delivery Group, which represents the industry, advised passengers in London and the South East to consider changing their travel plans on Thursday owing to the heat.
London North Eastern Railway (LNER), which runs inter-city services on the East Coast Main Line, is advising customers against travelling on Thursday.
It said some services were likely to be cancelled or delayed as speed restrictions will be imposed between Peterborough and London King's Cross.
— Met Office (@metoffice) July 24, 2019
The Met Office has warned heatwaves are on the increase as a result of climate change.
It is even possible the mercury could climb to 40C, which would be "unprecedented" for the UK climate, weather forecasters said.
The scorching temperatures gripping the UK and much of Europe come against a backdrop of global warming of 1C since the Industrial Revolution driven by greenhouse gas emissions.
Both Belgium and the Netherlands broke their all-time maximum temperature records on Wednesday, reaching 39.9.C and 39.1C respectively, the Met Office said.
Higher temperatures are making extreme hot spells more likely and more intense, experts warn.