The Met Office today announced an extreme heat warning for the first time in four years as a week-long 34C heatwave could bring the UK's hottest ever June day.Swathes of southern and eastern England and Wales were placed under the rare 47-hour amber warning which will run from 1am on Monday until the end of Tuesday.A static area of high pressure over mainland Europe is trapping warm air underneath - known as a 'heat dome' - and resulting in very hot conditions for southern England.Brits have been urged to 'keep out of the sun and avoid any exercise between 11am to 3pm when the sun is strongest and close curtains in rooms that face the sun'.They should also 'drink plenty of fluids' with the advice being: 'If you are going out take water with you, stay in the shade, wear sunscreen and a wide brimmed hat.'The Met Office said there is a 40 per cent chance of recording the highest ever June temperature, which stands at 35.6C both in London in 1957 and Southampton in 1976.It comes as concerns are mounting over another spike in water‑related incidents such as cold‑water shock and drowning, after at least 17 people died when they got into trouble in open water during the record-breaking May heatwave last month. Public transport users were told to consider 'amending travel plans if necessary' and 'carry water with you, take a small hand-held battery powered fan, if you feel unwell get off at next stop for some fresh air, dress in light fabrics in light colours to help stay cooler, keep an eye on fellow passengers for signs they may be struggling'.The heat could 'lead to widespread impacts on people and infrastructure' with 'some changes in working practices and daily routines likely to be required'.Meteorologists warned 'adverse health effects are likely to be experienced by those vulnerable to extreme heat', and 'some delays to road, rail and air travel are possible, with potential for welfare issues for those who experience prolonged delays'.There is an increased risk that 'some heat-sensitive systems and equipment may fail, leading to power cuts and the loss of other services to some homes and businesses'. Brits were told by the Met Office to prepare for possible power outages by 'gathering torches and batteries, a mobile phone power pack and other essential items'. Forecasters also fear that the 'wider population are likely to experience some adverse health effects including sunburn or heat exhaustion and other heat related illnesses.'The Met Office has only issued extreme heat warnings three times before – in August 2022, July 2022 – when the UK reached its only ever 40C day - and July 2021. The UK Health and Security Agency (UKHSA) has brought in separate amber heat health alerts for the South, where the heatwave is set to run until next Thursday. The Met Office has issued an amber warning for extreme heat for next Monday and Tuesday The Met Office forecast for London shows temperatures will soar beyond 30C next week Two women enter the River Thames next to the Millennium Bridge a hot day in London today People sit in the shade during a hot day four of Royal Ascot at the Berkshire racecourse todaySwimmers today at Sea Lanes Canary Wharf, a new floating swimming pool in East London Commuters cross London Bridge this morning as the sunlight is reflected from The Shard A beautiful morning at Gravesend Pier in Kent today where the Nao Victoria ship is visiting A woman jogging on The Queen's Walk in London this morning as the heatwave continues A man goes for a walk in the woods in the morning sunshine at Dunsden in Oxfordshire today The sun rises over Central London this morning as viewed from Richmond Park