The Central line is expected to be the hottest in London / ANDREW COWIE/AFP/GettyImages)

Parts of the London Underground could be hotter than the legal temperature for transporting cattle today and train delays are expected across the country

Temperatures could reach 35C today, making it the hottest day in almost a decade – and one of the worst commutes.

The heatwave has sparked speed restrictions on train lines across England amid fears tracks could buckle in the “extremely high temperatures”.

The precaution will make journeys in stifling carriages even longer on routes operated by First Great Western, Southeastern, Virgin Trains East Coast and Abellio Greater Anglia.

Meanwhile, parts of the London Underground are expected to be hotter than the legal temperature for transporting cattle.

As the mercury started to rise in this morning's rush hour, commuters took to Twitter to let off steam...

Some people really loved it though...

Employers have been urged to allow staff to travel outside rush hour to avoid overcrowded public transport during the exceptional temperatures.

Dr Angie Bone, head of extreme events at PHE, said the heat could be dangerous for older people, young children and those with serious illnesses.

Northolt in London saw the highest UK mainland temperature at 30.5C yesterday, while the Channel Island of Jersey hit 33C.

The highest July temperature on record was at Wisley, Surrey, in 2006, where the mercury peaked at 36.5C.

Thunderstorms could form in the Midlands and the North today before conditions cool slightly across the UK tomorrow.