Millions of people could be unknowingly shortening the life of their phone battery as temperatures soar, but experts say making one simple change during the heatwave could help keep it healthier for longer.

Another record tumbled on Thursday when the UK recorded its eighth day at or above 34C in a calendar year, breaking the previous high of seven days set in the summer of 1976 and in 2020.

With many parts of the UK set to bask in continuing hot weather this weekend, specialists have warned that one common habit can cause lasting damage to lithium-ion batteries, reducing how long they hold their charge over time.

It comes after Vodafone research found one in four people say the fear of their phone running out of battery is the most stressful part of their day.

"Basically, the worst enemy a battery can have is heat," Galo Paez Fajardo, a research assistant professor at the University of Warwick, told The i.

He explained that when a phone becomes hot, "those are signs that your device is telling you: 'Stop, cool it down', because when heat gets in is when the batteries accelerate the degradation."

That means leaving your handset on a sun lounger, car dashboard or table in direct sunlight could shorten its battery lifespan. Instead, experts recommend keeping it in a pocket, bag or shaded area whenever possible.

Scientists say the chemistry inside lithium-ion batteries speeds up as temperatures rise. Every 10C increase above around 25C can roughly double the rate at which batteries degrade. Charging a phone in temperatures of 35C to 45C can also cause extra wear, while a battery stored at 40C and fully charged could lose around 35% of its capacity in just a year.

As a result, experts have recommended making one simple change to help your phone last longer — avoid leaving your battery at 100% charge in hot weather.

"You don't want to be sitting at high states of charge at high temperatures," said David Howey, professor of engineering science at the University of Oxford.

Many newer phones include smart charging features that pause charging at around 80% before topping up closer to the time you usually unplug the device. Billy Wu, director of research at Imperial College London's Dyson School of Design Engineering, said: "Normally we say about 80 per cent charge is a good number to reach and doesn't accelerate the degradation too much."

However, Paez Fajardo said modern batteries have improved significantly, meaning it's often best to rely on your phone's built-in charging features rather than manually unplugging it.

Experts also dismissed the old belief that batteries should regularly be drained completely before recharging.

"Having your phone at zero per cent and 100 per cent are both high-stress states for your battery," said Adam Speight, senior researcher for product testing at Which?

Paez Fajardo added: "Now, current batteries are fine if you charge them at any percentage. Going all the way down to zero per cent is actually detrimental for some chemistries."

Fast charging remains useful but should be used carefully, especially in hot weather, as it generates more heat. If buying a new charger, Speight recommends choosing one that supports USB Power Delivery (USB PD), which manages charging more efficiently and reduces unnecessary stress on the battery.