Visitors at a country park have been warned people not to sit under trees as Britain braces for a fourth heatwave which will see temperatures reach the mid-thirties in some places.

Heat health alerts have been upgraded to amber by the UK Health Security Agency (UKHSA) for parts of the country as areas of southern England are set to meet their heatwave criteria on Wednesday, while other regions will breach the threshold on Thursday.

Temperatures could peak at 33C on Wednesday in the capital and surrounding areas, with some parts of the east Midlands and Yorkshire seeing 30C, according to the Met Office.

As the high temperatures have shown no sign of abating, staff at Normanby Hall Country Park in Lincolnshire have warned visitors that there is an increased risk of branches dropping suddenly as trees weaken in the heat.

While people may be used to enjoying the shade provided by trees, a parks and gardens officer at the 300-acre estate has warned the trees are under such stress that branches could shed without any warning.

"They're under a lot of stress at the moment," said parks and gardens officer Liam Dent, according to the BBC. "Due to a lack of water, the trees are basically going into survival mode. [They are] trying to protect the main trunk and [their] system. Any branches that are untoward or have cracks in them, they're just shedding them."

The park has asked visitors to “stay alert when walking beneath mature trees”, “avoid sitting, picnicking, or parking directly under large branches where possible”, “follow any temporary safety signage, barriers, or staff guidance in affected areas” and “report any fallen branches or concerns to a member of our team.”

The parks and gardens team will carry out routine inspections but has asked visitors to “remain vigilant and take extra care” while enjoying the park in the heat.

Normanby Hall is one of the region’s top visitor attractions, managed by North Lincolnshire Council.

UKHSA warned that the rising temperatures may increase health risks, particularly for vulnerable groups, as emergency services are braced for further wildfires during the next heatwave.

The country park’s warning to visitors comes a week after the National Drought Group heard that the high temperatures that have dominated this summer continues to have a “severe impact” on the environment.

The third heatwave of the year saw ponds and wetlands dry out, with some rivers drying up in places. The Environment Agency warned that a lack of rainfall in July, combined with the heat, was putting pressure on public supplies as with demand for water has soared up to 30 per cent on the hottest days.

England has only received 3 per cent of its long-term average July rainfall, with the south and east of the country receiving nothing at all so far – the longest spell without rain since the mid-1990s, they warned.