The train was due to terminate at Eastbourne.

The town's MP, Josh Babarinde, says he has already been informed of one constituent who suffered injuries to their ribs in the derailment.

In a social media post, Babarinde described the incident as "utterly shocking" and thanked emergency services for their response.

"I’m full of gratitude for our emergency services who have responded quickly and are pulling out every stop to ensure that passengers and staff who were on the train are taken to safety as soon as possible - thank you to them," he wrote.

"This is clearly distressing for those passengers and staff on board, even if they didn’t sustain physical injuries, and I have made representations for the appropriate mental health support to be made available to affected passengers and staff without delay."

He added serious questions will need to be answered about how this happened, but called for everyone to pull together in the interim.

BBC South East reporter Sofia Akin is near the scene of the derailment as emergency services continue their response.

East Sussex Fire and Rescue Service says it has sent 10 fire engines, a command support unit, technical rescue teams, officers and other resources.

A spokesperson said: "We have been helping reach passengers and assisting other emergency services."

Image source, Sofia Akin

We have been hearing from another passenger who was on board the train when it derailed near Lewes.

John May, from St Leonards, said the service from London Victoria was "quite packed".

"We were coming up here and suddenly the train just started to shift side-to-side and derailed," he told BBC News.

May said he was in the fourth carriage from the rear of the train.

"We were fortunate that we stayed on the embankment. The last three carriages rolled down into the field," he said.

He added: "There were a lot of older people and children."

University Hospitals Sussex NHS Foundation Trust has confirmed that it is currently responding "to a major incident" alongside other emergency services,

Southern says it has requested rail replacement buses to run between Haywards Heath and Lewes.

They have not yet been confirmed, so passengers have been told to continue using their own alternative routes.

There will be no service in either direction at Cooksbridge and Plumpton stations for the rest of the day. Passengers are being advised to speak to station staff or use station help points for further information.

Anyone travelling to Gatwick Airport and connecting to a flight is being told to allow an extra 150 minutes for their journey.

We have more from passenger Rob Bradley, who was travelling with his six-year-old daughter Mabel.

He says passengers remained on board for "about an hour and a half" after the derailment.

"It was quite pandemonium for a while. There's hundreds of police, fire and ambulance crews and everything," he told BBC News.

Bradley says it was "incredibly hot" on the train because the air conditioning stopped working and no doors or windows were open.

He adds that he heard police had counted 110 passengers across four carriages, although he says it was unclear how many people were on the rest of the service, which he described as busy.

These latest pictures from eyewitness Rob Bradley in Lewes show the aftermath of the derailment, as police, fire and rescue crews remain at the scene.

Rail industry expert Rich Wilcock says the immediate priority following the derailment near Lewes will be ensuring passengers can leave the train safely.

Wilcock, Industry Editor at Rail Magazine, told BBC News that emergency services would focus first on passenger welfare and securing the scene.

"The emergency services will be monitoring the situation in terms of safety for passengers and making sure that the line is safe for them to exit to a point of safety," he said.

He added that the RAIB will be sending a team to the scene, which it has since confirmed.

Wilcock also noted the 14:24 Southern service from London Victoria to Eastbourne may have been busier than usual because of the summer holidays.

Katy Austin
Transport correspondent

Photos showing what appears to be buckled track will prompt questions over whether that was a factor in this derailment.

The answer right now is that we can’t know for sure. It is not possible to be certain at this stage if the rails were already in that shape before the train arrived, or whether the bent shape is damage sustained due to the derailment.

However, it has been an exceptionally hot day, and it is known that rails can buckle in extremely high temperatures. Network Rail says the steel tracks can get 20 degrees hotter than the air temperature, and this can lead to them starting to curve.

Speed restrictions are one method that can be used as a precaution if there’s thought to be a risk of rails expanding.

Specialist investigators will look into exactly what has happened and why.

Aerial shots from the scene in East Sussex show the rear three carriages of the train left the tracks.

The affected service was the 14:24 Southern train from London Victoria to Eastbourne, which the BBC understands left the tracks in the Landport area of Lewes.

Verified image shows buckled tracks and passengers on top of derailed trainpublished at 18:06 BST

18:06 BST

By Richard Irvine-Brown

BBC Verify has confirmed that a picture of people standing on top of an overturned carriage was taken just north of Lewes station.

The image shows what appears to be a buckled track and at least four people standing on it.

We’ve checked features seen in the image - like the treeline off to the left of the shot - and can say the incident happened between Landport Road and the River Ouse about a mile north of the town’s railway station.

Image source, Jeremy Veitch

What we know:

• A passenger train has derailed in the area between Haywards Heath and Lewes in East Sussex shortly before 16:00 BST
• Photos from the scene show three carriages of the train on its side, with emergency service vehicles surrounding it and people standing on top of the sideways tilted carriages
• South East Coast Ambulance Service, East Sussex Fire and Rescue Service and British Transport Police are among the emergency services at the scene
• Network Rail says major disruption is expected until 22:00 BST, with Southern Rail saying no trains are running between Lewes and Brighton, Haywards Heath, Seaford and Eastbourne
• A passenger who was in carriage four of the train said it derailed from carriage five.
• Passengers on other trains which are not moving have been told by the operator to remain on their train and await further updates from the crew

What we don't know:

• What caused the train to derail
• The full extent of any injuries, although the BBC understands there have been no serious injuries or fatalities.
• Whether the incident will have any further impact on the wider rail network - including the length of any disruption and if more routes will be affected

Rob Bradley was on the train from London Victoria to Eastbourne with his six year old daughter Mabel.

"We are doing okay, we were very lucky we were on the fourth carriage. The fifth went on its side," he tells BBC News.

He says that 30 seconds before the derailment, his daughter was dancing between carriage four and five.

"As she sat down we felt the train speeding up and started to rock. Then our carriage jumped vertically upwards and we could tell it was a derailment. The emergency brakes went on the train and everything got thrown forward.

"The train skidded for quite a while before it stopped. We turned around and we saw the whole back of the train was open and smoke and dust were pouring into the carriage and we saw the fifth carriage had gone on its side."

He says a number of air ambulances flew in, and he saw people with head injuries, blood and cuts.

We have now heard from the Rail Accident Investigation Branch (RAIB), which is responsible for investigating incidents such as the Lewes derailment.

In a statement on X, it says it is "deploying a team of inspectors to the site of a passenger train derailment near to Lewes, East Sussex, to begin gathering evidence".

Southern Railway has told passengers "do not travel" in the affected area for the rest of the day.

This affects trains travelling on routes between Brighton, Lewes, Haywards Heath, Seaford and Eastbourne, with the majority of trains cancelled.

The affected train was the 14:24 Southern service from London Victoria to Eastbourne.

The TSSA transport union has said it is "deeply concerned" by the derailment near Lewes, adding its thoughts are with everyone affected.

It adds in a statement: "With a large number of passengers involved, their safety and wellbeing must come first."

UK train drivers' union Aslef has said it is aware of the derailment and asks the public to "allow the emergency services to do their job and refrain from speculation at this time".

Image source, Eddie Mitchell

New pictures sent into us show carriages have left the tracks of the Southern service.

A large emergency service presence can also be spotted at the scene.

Ben Motley
Live reporter in Lewes

I am standing on a hill above the town of Lewes and can see the train from a distance.

There is one carriage - the rear carriage - completely on its side at the bottom of the small embankment that the railway runs along.

There are two further carriages on their sides on the track.

We saw a number of people being brought away from the train in the searing heat. They have now walked on a dusty track into a housing estate.

I've seen lots of ambulances and fire engines.

There are three helicopters waiting in a field.

The BBC News helicopter is currently above the scene of the derailment, showing the current situation.

You can view the footage of the scene in the live stream at the top of the live page.