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Two people have been left seriously injured after a train dramatically derailed in East Sussex, shutting rail routes to and from London as emergency services rushed to the area.

Network Rail confirmed a Southern Rail service diverted off the tracks in Lewes just before 4pm on what is the hottest day of the year so far, with temperatures in Haywards Heath hitting 32C.

Extreme temperatures can cause lines to buckle and, prior to today's crash, eight national operators had warned of disruption because of warm weather.

Authorities have ordered the public to avoid the vicinity as medical professionals and firefighters respond to reports of passengers being trapped.

The train's journey started at London Victoria and was heading to the town of Eastbourne on the south coast.

It is understood three of the British Rail Class 377 Electrostar's eight carriages overturned.

Three air ambulances arrived alongside around 20 other emergency vehicles.

Witness Ross Taylor told the Brighton Argus: “I saw the carriages on their side... I have never seen a response like this.”

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“It is quite nuts... the blue lights everywhere, the helicopters, but they don’t understand the severity of it.”

Rob Bradley, who was on the train with his six-year-old daughter, told the BBC: “We felt the train kind of speeding up, and then starting to rock, and at that point I knew something was very strange.

"Then our carriage actually jumped vertically upwards and we could tell it was a derailment. The emergency brakes then went on the train and everything got thrown forward — you know, my laptop, my AirPods, our bags.”

Thankfully, Government sources confirmed the derailment resulted in zero deaths or serious injuries.

As a result of the derailment, no trains can currently run between Lewes and Brighton, Haywards Heath, Seaford or Eastbourne, with major disruption to the network south of London, including shuttles to Gatwick Airport, until at least 10pm.

National Rail said: “You will need to use an alternative route to reach your destination.

“If you are already travelling or must travel, you should allow at least an additional 60 minutes to complete your journey today.”

An East Sussex Fire and Rescue spokesman said: "We can confirm we were called at 3.55pm to reports that a train had derailed just outside Lewes.

"We continue to work at the scene alongside our emergency service colleagues and other partners."

South East Coast Ambulance Service says there were about 150 people on board the train when it derailed near Lewes.

A spokesperson said: "We can confirm that two patients have sustained serious injuries and a further nine patients have sustained less serious injuries.

"Patients have been assessed and treated at the scene, with some transported to local hospitals for further treatment and care.

"A number of passengers who did not require hospital treatment have been taken to a local community centre, where they are being supported by partner agencies.

Heidi Alexander, the Transport Secretary, said she was "deeply concerned" by images surfacing on social media.

She stated: "Grateful to emergency services who are on the scene supporting those impacted."

James MacCleary, the Lewes MP, said: “If you are concerned about someone or need urgent assistance, please contact the emergency services.”

Meanwhile, Helena Dollimore, the MP for nearby Hastings and Rye who spoke to Ms Alexander, urged her constituents to contact her office if they need “urgent support”.