Britain has experienced the hottest day of the year so far, and forecasters predict the heatwave will continue.

After a weekend of sunshine around the UK, temperatures in St James’s Park peaked at 29.4C (84.9F) on Monday, in line with sunny holiday hotspots in southern Spain. The pollen count and UV levels are also very high. The previous highest temperature recorded in the UK was 29.1C at St James’s Park on 19 April.

The Met Office issued a heatwave alert adding that there was an 80% chance of heatwave conditions between Tuesday morning and Thursday evening in parts of England.

Under Public Health England’s heatwave plan, when temperatures exceed about 30C, children are advised not to take part in vigorous exercise and are encouraged to stay in the shade as much as possible.

Temperatures that trigger heatwave alerts vary by region, from 28C during the day and 15C at night in north-east England, to 32C in the day and 18C at night in London. The Met Office’s heat-health watch said some areas of north and west England were likely to be hotter.

But temperatures are not expected to increase by quite as much as previously expected. The Met Office alert said: “While very warm or hot weather is also expected in the other regions, temperatures are currently not expected to reach thresholds in these locations until at least Thursday.

“Eastern parts, especially along the coast, are also likely to see cooler temperatures than the rest of the country due to an onshore breeze along with the risk of some patch mist or low cloud moving in from the North Sea at times.”

Dr Thomas Waite, of Public Health England, said: “We’re urging everyone to keep an eye on those you know who may be at risk this summer. If you’re able, ask if your friends, family or neighbours need any support.”

PHE has issued tips for staying safe in the heat, including:

Look out for others, especially older people, young children and babies and those with underlying health conditions.
Close curtains on rooms that face the sun.
Drink plenty of water as sugary, alcoholic and caffeinated drinks can make you more dehydrated.
Never leave anyone in a closed, parked vehicle, especially infants, young children or animals.
Try to keep out of the sun between 11am and 3pm.
If you are going into the water to cool down, take care and follow local safety advice.
If you have to go out in the heat, walk in the shade, apply sunscreen and wear a hat.
Avoid physical exertion during the hottest parts of the day.
Wear light, loose-fitting cotton clothes.
Make sure you take water with you if you are travelling.

The Met Office is also warning of very high levels of pollen and ultraviolet light. The forecaster Mark Foster said the heat could set a new high for the year: “There is a fairly good chance we will see the hottest day so far … There is a chance it could possibly be tomorrow [Tuesday].”

The highest temperature recorded in the UK so far this year is 29.1C in St James’s Park, central London, on 19 April – the warmest April day for almost 70 years.

Foster added: “High pressure is going to dominate this week, so we can expect very high levels of pollen and UV. If you are in the sun, you have a greater chance of getting burnt in the short term.”

Network Rail warned that tracks could buckle in the heat, while the AA pointed out that asphalt on roads begins to soften at 27C, making roads sticky and potentially dangerous as cracks and bulges emerge.

Jack Cousens, the head of roads policy for the AA, said: “While many drivers will seek some fun in the sun, our road network will be desperate for some shade.

“Drivers may get a surprise as the gritters make a return to our streets. However, rather than spreading salt, they will be dusting sand on the roads as a form of sunscreen for the tarmac.”