A third heatwave in as many months has begun in the UK, with temperatures set to hit 35C later this week. The Met Office said on Monday evening that south east England had officially met the heatwave criteria, after three days of temperatures above 28C.The highest temperature on Monday was recorded as 34C in Teddington, south-west London. Elsewhere in the capital, temperatures hit 33.3C in Heathrow and 32.7C in Kew Gardens.May and June both saw heatwaves, with temperatures reaching a high of 37.7C last month. That beat the previous June record dating back from the summer of 1976, and combined with high humidity and “tropical nights”, where temperatures do not drop below 20C.How hot will this heatwave be?Temperatures are set to continue rising across the UK throughout this week, and more regions are likely to reach their local heatwave thresholds, the Met Office said.Parts of southern England could see temperature highs of about 32C on Tuesday, rising to 33C on Wednesday and 34C on Thursday, with the highest readings of 35C forecast for isolated areas on Friday and Saturday.A north-south divide will persist through the first half of the week, according to the forecaster.Scotland should see stronger winds and outbreaks of rain on Tuesday, with temperatures remaining generally closer to the average for this time of the year.But, by the weekend, the warm air is forecast to reach northern parts of the UK, and conditions are set to become increasingly humid, with a growing risk of showers or isolated thunderstorms.When will the heatwave end? The very warm conditions are likely to last into the weekend, the Met Office said, but with increasing humidity and a growing risk of showers or isolated thunderstorms.Into early next week, temperatures are likely to trend down slightly, although many areas will stay dry, fine and still warm.Amber heat health alerts have been issued by the UK Health Security Agency (UKHSA) for the Midlands and southern England, from 9am on Wednesday until 9pm on Sunday.This means significant impacts are likely across health and social care services due to the high temperatures, including a rise in deaths, particularly among those aged 65 and over or with health conditions.People shelter from the sun in Ely Park in Cambridgeshire on 26 June, during the intense heatwave that sent temperatures soaring in much of the UK (PA Wire)Yellow heat health alerts have also been issued by the agency for Yorkshire and northern England, also between 9am on Wednesday to 9pm on Sunday.The UKHSA said similar significant impacts are possible across these areas, and warned of a potential increase in water-related incidents across England, including risks from cold water shock and drowning.What will the impact of this heatwave be?The Met Office’s deputy chief forecaster, Steven Keates, said people should “take the usual precautions with the heat and the sun”.He said: “Parts of the UK are entering heatwave conditions: the third heatwave in the UK so far this year.“However, unlike the May and June heatwaves, we are not expecting this heatwave to be record-breaking.“Temperatures this week are not expected to reach the highs we witnessed last month, though parts of southern England in particular are likely to see several days in the low 30s, and a few places could reach 34-35C later this week.”The Met Office also warned England could experience tropical nights later this week.National Rail Enquiries has urged train passengers to check for updates over the coming days because of the heatwave.Vehicle breakdown rescue companies are also likely to be busier than usual because of the high temperatures.Drivers are advised to carry plenty of water and conduct essential vehicle checks before setting off, particularly tyre pressures and their levels of coolant and oil.A man holds two pints as he walks next to the River Great Ouse in Ely during the last heatwave (PA)Tony Rich, a spokesman for the AA on road safety, said: “With temperatures expected to climb into the low 30s across parts of England over the coming days, drivers should take the heat as seriously as they would ice or heavy rain.“A few simple checks before setting off can make all the difference. Make sure your tyres are correctly inflated, check your coolant level and carry plenty of drinking water for everyone in the vehicle, including pets.“If you’re planning a longer journey, build in regular breaks and try to avoid the hottest part of the day where possible.“Should you break down, get yourself and your passengers to a place of safety, ideally behind the safety barrier if there is one, and keep hydrated while waiting for assistance.”The Met Office said UV levels are expected to be high or very high across many parts of the UK throughout the week, and advised people to ensure they use sun protection.The forecaster further warned pollen levels are also expected to be high or very high in some areas this week.