Britain's third heatwave of the year could be its longest on record as temperatures intensify to 36C today and rail passengers were urged not to travel unless 'essential'.Parts of south-east England have now officially been in a heatwave since Saturday and forecasters believe the threshold will continue to be met well into next week.The conditions could yet be record-breaking, given the historic 1976 heatwave lasted for 16 days in a row with at least 30C in London each day from June 23 to July 8.The current heatwave is expected to peak today and tomorrow with highs of 36C in the South-East before easing slightly to 30C this weekend and 28C early next week.But these temperatures will still be enough to hit the heatwave level, which varies around the UK but is 28C in London and must be recorded for at least three days.The Met Office said conditions will turn slightly cooler but still dry from next week in the South-East due to an easterly wind, while the focus for the hottest conditions will shift towards the South-West where there could also be thunderstorms.NHS England warned that many of its ambulance services would be under 'sustained pressure' due to the heat which it has described as 'dangerous' for some people. It comes after experts revealed western Europe experienced its hottest June last month – and the UK had its hottest ever May and June temperatures this year. Meanwhile rail passengers continued to face disruption due to the heat across the network as East Midlands Railway (EMR) issued an 'essential travel only' warning.Customers were urged to travel before 12pm to avoid the highest temperatures, with regional services between Nottingham and Worksop and Nottingham and Leicester being suspended from that time to reduce the impact of the heat on the railway.Great Western Railway, c2c, London Northwestern, South Western Railway Transport for Wales and West Midlands Railway were also running amended services. Swimmers cool off from the hot weather today at London Fields Lido in Hackney, east London People wait in the hot weather for the arrival of the King and Queen at ZSL London Zoo today Swans on the River Severn in Worcester today as temperatures are set to continue to rise A woman uses an umbrella for shade near London Euston station during the heatwave today A swimmer cools off in the Sky Pool at Embassy Gardens in London today to escape the heat A beautiful morning in the woods at Dunsden in Oxfordshire today as the heatwave continues A woman jogs around the lake at Sefton Park in Liverpool during the very hot weather today The Met Office forecast for London shows the 28C heatwave threshold being met every dayHigh temperatures can cause rails to buckle, overhead electric wires to sag and lineside fires, while steel rails can exceed 50C when the air temperature is 30C.UK Health Security Agency (UKHSA) amber heat health alerts will remain in force for the Midlands and southern England at least until Sunday, meaning 'significant impacts' are likely across health and social care services because of the heat.Officials warned of a 'rise in deaths', particularly among those aged 65 and over or with health conditions; while yellow alerts were issued for northern England. TOP TEN HOTTEST DAYS IN BRITAIN - INCLUDING TWO IN JUNE 20261 40.3C July 19, 2022 Coningsby, Lincolnshire 2 38.7C July 25, 2019 Cambridge 3 38.5C August 10, 2003 Faversham, Kent 4 38.2C July 18, 2022 Pitsford, Northamptonshire 5 37.8C July 31, 2020 London Heathrow 6 37.7C June 26, 2026 Lingwood, Norfolk 7 37.1C August 3, 1990 Cheltenham, Gloucestershire 8= 36.7C August 9, 1911 Raunds, Northamptonshire 8= 36.7C July 1, 2015 London Heathrow 8= 36.7C June 25, 2026 Merryfield, Somerset Met Office deputy chief Forecaster Tom Crabtree said: 'Many people will understandably want to know how long the current heatwave is likely to last.'While temperatures are expected to ease across south-east England through the weekend and into next week, it won't be a straightforward end to the warm weather.'High pressure is expected to remain in charge through much of next week, but it will gradually migrate northwards. This means that an easterly wind will become established in the south, and the focus for the hottest conditions will migrate towards the South-West.'As a result, some places that have seen the highest temperatures during recent weeks will begin to cool. It will however remain very warm, especially in the South-West, and heatwave thresholds may continue to be met in places.'Temperatures will widely exceed 30C across England and Wales today, with highs of up to 36C today and tomorrow. Saturday could see up to 34C in south-west England and 33C on Sunday, before a gradual trend down next week.In Scotland and Northern Ireland, 28C is possible tomorrow, with the mid-20Cs this weekend.The heatwave has now been running for five days – with UK highs yesterday of 33.8C in Wisley, Surrey; and 33.7C at London Heathrow Airport. Firefighters dealt with a large wildfire after sparks from a piece of flint struck by farm machinery ignited tinder dry crops at Kingston Russell, near Bridport in Dorset yesterday, More than 30 firefighters from Dorset and Wiltshire Fire Service rushed to the scene of the blaze in Kingston Russell yesterday and spent several hours tackling the inferno Amber heat health alerts have been issued by the UK Health Security Agency (UKHSA)Prior to that, London was the hottest place in the UK for four days in a row – with 32.4C in Teddington on Tuesday, 34C in Teddington again on Monday, 30.3C at Heathrow on Sunday, and 29.3C in Teddington yet again on Saturday.In Dorset, firefighters have dealt with a large wildfire after sparks from a piece of flint struck by farm machinery ignited tinder dry crops.A farmer had been working in a field at Kingston Russell near Bridport when the blaze broke out at 1pm yesterday. Hosepipe bans and requests from firms Southern Water: Hosepipe ban for one million people across Hampshire and the Isle of Wight, in force from July 10South East Water: Hosepipe ban for 850,000 people in Kent, in force from today. People in Hampshire and Sussex asked to reduce water useThames Water: Customers asked to stop using hosepipes during heatwaves but no ban in placeAnglian Water: No hosepipe ban, but customers asked not to use them to conserve water - and firm says ban cannot be ruled out for this summer More than 30 firefighters rushed to the scene and spent several hours tackling the inferno which destroyed 30 acres of crops.A spokesperson for Dorset and Wiltshire Fire Service said: 'It is believed the fire started after farm machinery struck flint, creating sparks that ignited the dry crop.'At this time of year, farmers work long, hard hours and it's easy to let chaff and dust build up in the machines, which can then catch fire if any parts then overheat.'The problem can be compounded when machinery is worked extra hard during hot weather.'The National Fire Chiefs Council (NFCC) has issued a warning on the increasing risk of wildfires because of the dry and hot weather - advising people to avoid using disposable barbeques in open countryside, park and moorlands.Dave Swallow, the NFCC's wildfire deputy lead and lead tactical adviser, said: 'Hot, dry weather can significantly increase wildfire risk, but most wildfires start because something provides the spark.'Whether it is a disposable barbecue left behind, a discarded cigarette or even a glass bottle left in the sunshine, we all have a role to play in preventing them.'Summer should be a time for people to enjoy the outdoors, but it's important not to underestimate the risks. Every year we see devastating wildfires and tragic drownings in inland water.'As more schools break up for the summer holiday, we're asking people to enjoy the warmer weather safely, look out for one another and take simple steps to help prevent avoidable tragedies.'Surrey Fire and Rescue Service said crews had dealt with 'several' wildfires this week, including at Whitmoor Common where discarded glass caused a blaze which burned an area equivalent to around one and a half football pitches.