
Amber heat health alerts have been extended to the north of England, as the third heatwave of the year saw temperatures soar to 35C (95F) on Thursday.
The alerts cover almost all of England, indicating that the heat is likely to have a significant impact on health services and cause possible travel disruption until 21:00 BST on Sunday.
NHS England has warned many of its ambulance services are facing "sustained pressure" due to the heatwave, and rail services have advised passengers to expect disruption.
While temperatures are not due to match those recorded in June's record-breaking heatwave, the heat may last longer - possibly until the end of next week.
The current heatwave is not expected to break any daily temperature records - but earlier the Met Office announced is is the eighth day this year where the heat has reached or exceeded 34C.
There were two in the May heatwave, four in June's and so far two in this one, which is a record for the calendar year, surpassing 1976 and 2020. A further temperature reading of 34C or over is expected on Friday.
The alerts for north-west England and Yorkshire - issued in situations that could potentially put the whole population at risk - join those already in force for the Midlands, eastern, and southern England.
A yellow alert, indicating that significant impact is possible, covers the north-east.
The worst of the heat is expected in southern and eastern England, parts of which declared a heatwave on Monday after five days at or above 30C, placing additional pressure on players and spectators at Wimbledon gearing up for the semi-finals.
Temperatures hit 35.1C in Wisley, Surrey on Thursday afternoon - approaching the record 35.7C seen at the London tennis championship in 2015.
Additional water and electrolytes are being provided to players, as well as iced towels and umbrellas during changeovers. A 10-minute break can be requested between sets if the temperature and humidity pass a certain threshold.
Meanwhile on Thursday, 30.4C has been recorded in Cardiff, Wales, 27.0C in Aboyne, Scotland and 25.0C in Killowen, Northern Ireland.
Rail passengers have been advised to expect disruption with the heatwave expected to peak on Thursday and Friday.
Several c2c trains have been suspended for the next two days, while East Midlands Railway has advised passengers to avoid all but essential travel.
West Midlands Railway and London Northwestern Railway reported "heat related issues" affected some of their services early on Thursday. Operators South Western Railway and Great Western Railways have warned they are expecting to face similar disruption.
Transport for Wales has said a speed restriction is in place due to high track temperatures between Aberdare and Fernhill in South Wales, with disruption expected until 21:30.
Separately, the UK's electricity grid operator Neso has issued a request for more power to be made available during the ongoing heatwave.
Extreme heat can make energy processes less efficient and lead to a spike in electricity demand as people increase their use of air conditioning, freezers and fridges.
Neso's notice warns there could be "tight electricity margins" on Thursday evening, but does not suggest a risk of shortages or blackouts.
Meanwhile, the public has been advised to heed safety advice and care for each other as the prolonged spell of extreme heat continues.
NHS England has asked the public to "take the heat seriously" - particularly when it comes to children, the elderly and those with existing conditions - as it warned many of its ambulance services were under pressure.
The Royal College of Paramedics advised drinking enough water, supporting those who may be vulnerable with access to water and cooling systems, and trying to stay out of the sun when the heat tends to peak from 12:00-16:00.
The Welsh Ambulance Service has warned dehydration and fatigue from the heat could cause falls, while South Wales Fire and Rescue has advised extra caution around open water.
"Warm weather does not mean warm water, and cold water can affect your breathing and swimming ability very quickly," cautioned station manager Gavin Murphy.
The National Fire Chiefs Council (NFCC) warned on Wednesday that the dry and hot weather posed an increased risk of wildfires. The NFCC also advised people to avoid using disposable barbecues in open countryside, park and moorlands.

