An official code of practice introduced in 1992 as part of the Workplace (Health, Safety and Welfare) Regulations states that workplace temperatures should not normally drop below 16C - or 13C if the work involves severe physical effort - but there is no suggested maximum limit, with the guidelines stating only that 'all reasonable steps should be taken to achieve a reasonably comfortable temperature'.

The TUC has backed a legal maximum temperature of 30C - or 27C for those doing strenuous work - and Ms Riordan’s motion urges the Government to adopt these levels in law.

As the warm weather continues, the Met Office issued a level three 'heat health watch' for London and the South East and advised 'alertness and readiness' for those in the North West and North East.

Level three warnings are only one notch below the most serious alerts and are put out when high temperatures place the very young, the very old and those with chronic diseases at risk. 

Yesterday a man and a boy, aged 15 and 40, were found dead in a lake where swimming is banned after apparently jumping in to cool off as Britain basks in the first heatwave for seven years.

The pair went missing yesterday afternoon after being spotted in the water at the Bawsey Pits country park near King's Lynn, Norfolk. Police officers, firefighters and paramedics launched a search for them and their bodies were recovered from the lake last night.

A Norfolk Police spokesman said: 'Both bodies were discovered within the water and inquiries continue into the circumstances surrounding their deaths.'
Police have asked people to stay away from the area.

Although swimming is banned at the beauty spot, which is popular with sunseekers, but many ignore warning signs.

The dangers of drowning increase in the warm summer months as people try to cool themselves and there have been several tragic incidents in recent weeks.

In Cambridge, a couple were found dead on the banks of the River Cam yesterday just a short distance from families enjoying the warm weather.

The man and woman were declared dead at the scene and a third person was taken to the city's Addenbrooke's Hospital for treatment. It is believed all three had been drinking by the riverside as part of a group since the previous evening and had taken drugs, though it is not known what substance was involved.

Police said there were early indications the deaths were drug-related but officers refused to rule out the searing 29C heat as a contributory factor.

Today temperatures could creep as high as 32C in parts of England, making it the warmest day of the year so far. It means Britain will bask in its fifth consecutive day of 30C-plus summer weather - the first prolonged 'heatwave' since 2006.

Last Saturday 31.9C, or 89.4F, was recorded in Southampton, which was the hottest July day in seven years.

Gemma Plumb, forecaster with MeteoGroup, said: 'There is no proper definition of "heatwave" in the UK - but we are undoubtedly in the midst of one now.

'It is the longest period of hot weather since 2006, with prolonged temperatures above the average for the time of year. Normally we can expect it to reach 20-22C in England and Wales - today parts of the Midlands, East Anglia, the south coast and London could see 30C-plus.'

Weather warning: The Met Office this morning issued a level three alert - which means there is a 90 per cent chance of heatwave conditions

And sun-worshippers can expect to enjoy balmy Britain for a while yet - with the rest of the week expected to vary from 25C to 31C.

Miss Plumb said: 'At the moment it looks as though the warm weather is going to stay well into next week at least.'

In Newcastle, young troublemakers have started to open up fire hydrants in a bid to cool down in the hot weather, police said.

Likening the scenes to something more common in downtown New York, Northumbria Police said lives are being put at risk. But they warned that opening hydrants is a criminal offence and they will arrest anyone caught doing it.

Police Inspector Mark Farrimond said: 'It may all seem like fun and games, but fire hydrants are there for a very good reason - to save lives.

'Opening them up is a criminal offence and we take it very seriously. Earlier this month we arrested a 19-year-old man after a hydrant was opened on Belgrave Parade in Elswick.

'As well as affecting water supplies and causing local flooding, opening up such hydrants causes real problems for firefighters.'

The fire service said that if crews were unable to access a hydrant in the event of a blaze, people could lose their lives.

As the hot weather continues, the Met Office ramped up their heatwave warning for London and the south-east from level two to level three, one step below the maximum.

Across much of the rest of England a level two alert is still in place.

The system is overseen by Public Health England, a Department of Health agency, in a bid to reduce the health impact of the current run of stifling temperatures across the UK.

A level three warning is triggered as soon as the Met Office forecasts that there is a 90 per cent chance of temperatures being high enough on at least two consecutive days to have significant effects on health.

Death rates rise soon after temperatures soar, with many fatalities occurring in the first two days.

Level three is triggered as soon as the Met Office confirms that a threshold temperature has been reached in any one region or more.

These vary according to the area, but the average temperature is 30C during the day and 15C overnight.

A decision on what level of warning is in place for each of nine English regions is made every morning from the beginning of June to mid-September by a team of forecasters in the Heat Health Watch Service at the Met Office.

Beach weather: Rachel Fairhurst from Cornish Sea Horse walks her horse Bella, down to the seafront at the Long Rock Beach in Cornwall

Today two regions of England are at level three, five at level two and two at level one. Wales is also at level one.

Dr Angie Bone, Heatwave Plan lead for PHE, said: 'In this continued hot weather, it’s important to remember that high temperatures can be dangerous, especially for people who may be vulnerable such as older people, young children and those with serious illnesses.

'During very hot weather, pregnant women and people who have chronic illnesses, including cardiovascular, respiratory, renal conditions, diabetes or Parkinson’s disease, may experience discomfort if indoor temperatures are particularly hot and in using public transport.

'Employers should ensure indoor areas are kept cool and consider allowing these individuals to travel to or from their place of work during cooler, or less busy, times of the day. For those working or exercising outdoors, strenuous physical exertion during the hottest part of the day should be kept to a minimum.