London’s heatwave is expected to hang on until next week, with temperatures in large swathes of southern England and Wales forecast to hit 35C on Tuesday and creep up to 36C in places. The mercury is then set to fall back to 22C with rain on June 1, the first day of cooler, wetter weather since the UK recorded its hottest May day of all time.

The Met weather outlook leaves little doubt that Tuesday, May 26, will be the peak of the spell for many people. Forecasters said the searing conditions are being driven by warmth building under an area of high pressure near the UK, and the heat will be felt widely enough to keep the day uncomfortable well beyond the south-east.

The pressure system is doing the heavy lifting. Once it settles in, it traps hotter air and keeps the atmosphere dry, which is why the forecast for Tuesday remains so extreme. Dan Suri, a forecaster, described the setup as “the influence of warmth building under an area of high pressure near the UK,” and that is exactly what is pushing temperatures into territory more often associated with the Mediterranean than much of Britain.

That does not mean the turn will be abrupt. Temperatures are expected to start gradually declining from the middle of the week, but the weather will stay largely dry with sunny spells as the heat eases. Many people will still see high 20C readings, and the drop will feel more like a step back than a full retreat.

By Wednesday, June 3, the forecast points to a high of 20C and a low of 13C, with light rain showers and a moderate breeze. Thursday, June 4, is expected to be similar, with 19C by day, 12C overnight, light rain showers and a moderate breeze. Friday, June 5, should edge back to 20C, with a low of 13C, thundery showers and a gentle breeze. The has also forecast thunder next Friday, May 5, underscoring how unsettled the late stretch of the outlook could become.

There is one more complication for anyone hoping to enjoy the heat while it lasts: Tuesday is also expected to bring a very high pollen count. For people already dealing with hay fever, that means the hottest day of the week may also be the hardest to breathe through.

The broad shape of the forecast is now clear. Britain is in for one more blast of intense early-summer heat, then a slower, wetter reset as June begins. The heatwave is not ending at once, but it is already on the way down.