THE temperature has topped 33.9C today, making it the hottest June day since 1976, the Met Office says.
The record was broken at Heathrow in west London at lunchtime today.
Temperatures are at their highest for a June day since the 35.6C (96F) recorded in Southampton on June 28 1976, and it is the warmest summer solstice on record.
The 40-year high has been recorded as parts of Britain swelter in the fifth consecutive day with temperatures above 30C (86F), in what is the hottest prolonged spell in June since the drought summer of 1976.
But there are also weather warnings in place for Wednesday afternoon and evening, with heavy rain and thunderstorms forecast for parts of southern Scotland, northern England, north Wales and the Midlands.
The Met Office warned of the potential for torrential downpours, frequent lightning, very large hailstones and strong gusts of wind, which could lead to localised flooding and temporary disruption of power supplies.
The heatwave is set to come to an end on Wednesday, as a cold front sweeps across the UK overnight.
Chief meteorologist Steve Willington said ‘The high pressure that has dominated our weather of late is starting to move away, allowing fresher air in from the west.
‘A cold front that will pass through the UK will mark an end to the hot spell of weather in the south and bring cloudier skies and lower temperatures.’
The sweltering temperatures have seen ‘unprecedented demand’ for ambulance services in London, with people fainting, collapsing and becoming unconscious in the heat.
Patients calling for non-emergencies are likely to wait four hours for an ambulance, London Ambulance Service warned.
On Monday, London Ambulance Service call handlers answered 6,613 emergency calls, compared with 4,695 the week before - a 41per cent increase - and the service warned this was expected to continue while the heatwave lasted.
Peter McKenna, deputy director of operations, said: ‘Our crews are extremely busy. On Monday we attended 20per cent more seriously ill and injured patients than the same day last week and we’ve also been involved in a number of high-profile major incidents.’
Medical director Dr Fenella Wrigley said: ‘We see an increase in calls because people can forget to stay hydrated and the heat can exacerbate heart and breathing conditions.
‘We are getting calls from people who do not need an ambulance - for minor sunburn, heat rash, hayfever. These can be dealt with by a pharmacist. If you call us for something minor, you may experience a long wait.’
Ambulance chiefs are warning people to only call for life-threatening or serious emergencies and to check on vulnerable friends, family members and neighbours such as older people who are more at risk.
People enjoying the sunshine are also advised to drink plenty of water, stay in the shade to keep cool, carry essential medication with them, cover up and drink alcohol in moderation.