Firefighters in the UK are under increased pressure as a result of an “unprecedented summer of dangerous and volatile wildfires”, the Fire Brigades Union has said, as many parts of the country brace for a fifth heatwave of the year.

As authorities declared almost three-quarters of England was now in drought, Steve Wright, the FBU general secretary, said the unpredictable environmental challenges the UK was now confronted with meant it was a “national priority” to have enough properly equipped firefighters to “face these threats”.

More than 120 firefighters continued to tackle a blaze in the New Forest national park in Hampshire that began on Sunday. Nottinghamshire fire and rescue service declared a major incident on Sunday as two large fires in woodland near Mansfield put a significant strain on resources there.

Emergency calls to the London fire brigade have increased by almost a third this summer as firefighters have battled a surge in wildfires and flooding in the capital, according to an internal briefing seen by the Guardian.

The document warned that the brigade had come under mounting pressure from extreme weather and predicted the trend would continue because of the climate crisis. Between 1 April and 9 July, the service received 75,652 emergency calls – 31% more than the previous five-year average – while the number of incidents rose by nearly a quarter to 42,943.

London firefighters responded to 392 wildfires during the period, alongside 68 water rescues, the briefing said, as repeated spells of hot weather and heavy thunderstorms stretched the service’s resources.

The climate breakdown has been linked to the increasing frequency of wildfires, burning larger areas, across many parts of the world. The so-called “fire wave” under way in the UK is the most widespread the country has ever seen, with the combination of wet winters and prolonged heatwaves exacerbating the risk.

In recent weeks, there have been large blazes in places including Dunwich Heath in Suffolk, Headon Warren on the Isle of Wight, Llanwonno in the Rhondda valley in south Wales, and the Cairngorms national park in Scotland.

The National Drought Group, which met on Monday, said 71.3% of England was now experiencing a “flash drought”, which develops very quickly brought about by a combination of very low rainfall and higher temperatures. About 45 million people live in a drought area and more than 27 million are now living under water use restrictions.

The group said all regions of the country were experiencing pressure on their water resources, with growing pressure on agriculture, public supplies, navigation and the environment.

Tony Grayling, the chair of the National Drought Group, said: “Drought conditions are worsening and will continue to do so until we see significant and sustained rainfall. We know the hot and dry weather is taking a real toll on farming and there will be long-lasting impacts on our environment.”

He urged everyone to use less water to “make sure our current water reserves last”.

No 10 said the dry conditions were having a “significant impact on communities, farmers and businesses across the country”, adding: “Our climate is changing. Droughts are becoming more common.”

The prime minister’s spokesperson said the government was focused on “both the immediate response and long-term resilience”, including working through the National Drought Group to “protect water supplies and minimise environmental damage”.

The spokesperson did not say what threshold would constitute a national emergency. Ministers were monitoring the situation closely, No 10 said, and the Environment Agency had stepped up monitoring and was working with water companies, farmers and local partners to “manage pressure on water resources”.

Joanne Smith, the LFB’s deputy assistant commissioner for control and mobilising, said the service had responded to thousands of emergency calls and “hundreds of wildfires alongside water rescues and other significant incidents across the capital”.

She said: “We have learned from previous events such as the summer of 2022 and developed our training for firefighters and officers to enhance their knowledge of tactical awareness at wildfires. We’ve invested in new equipment and training to help tackle wildfires, and we will continue to evolve so that we are ready and prepared for future summers.”

The FBU’s Wright called for “increased funding, improved access to the best equipment and better national resilience” to combat rising demands on firefighters. He added: “Our country is facing unpredictable environmental challenges, and it must be a national priority to ensure we have enough properly equipped firefighters ready to face these threats.”

He has previously said fire crews had been stretched by staffing cuts, with 12,000 fewer firefighters in the UK today than in 2010.

In Hampshire, hundreds of people were forced to abandon their vehicles and walk after Sunday’s wildfire caused by a van fire on the A31 near Ringwood spread to nearby New Forest heathland and affected an area the size of 140 football pitches.

Twenty-six fire engines and four water carriers were called to the scene, which was scaled down to 10 engines and 13 off-road vehicles by Monday.

Hampshire fire and rescue said on Monday: “There are more than 120 firefighters on the scene battling the heat and arduous terrain in order to prevent further fire spread. We are monitoring the fire, which is affecting a 1 square kilometre area of heathland off the A31, with police drones.”

Amber heat health alerts have been issued for the East Midlands, West Midlands, east of England, Yorkshire and the Humber, London, the north-west, south-east and the south-west, while a yellow heat health alert has been issued for the north-east, from Tuesday at 9am until Friday at 9pm.

The Met Office said it was expecting temperatures to exceed 36C in some areas later in the week, after a relatively cool day on Monday, with the heat peaking on Thursday. England and Wales have had their driest July in records dating back to 1836, with 19 counties of England seeing no rain for the entire month, provisional statistics show.

Southern England had its hottest July on record, and it was the joint hottest July on record for Wales, while England and the UK as a whole had their second hottest July, Met Office figures show.

A spokesperson for the mayor of London said: “The prolonged, extreme heat is presenting incredible challenges for our emergency services, with a huge rise in demand.” They said the mayor had provided record funding to ensure LFB was better equipped and to improve training.

Additional reporting by Aletha Adu