A train has derailed near Lewes in East Sussex, British Transport Police say
The force says it is "responding alongside paramedics and the fire service" - the full statement
An image from the scene shows people stood on top of train carriages, which are tilted to the side - you can watch live images of the scene at the top of this page
We have seen people being escorted away from the train down a dusty path, as three carriages are on their side, writes our reporter at the scene
A source from the Department for Transport tells the BBC it understands there are "no serious injuries or fatalities", our transport correspondent reports
Southern Rail says major disruption is expected until 22:00 BST as trains are unable to run through the area
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 MitchellNew 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.
A helicopter above the scene shows the derailment
Katy Austin
Transport correspondent
A source from the Department for Transport has told the BBC that the latest understanding is that there are "no serious injuries or fatalities".
Lewes Railway Station sits near the town centre and connects it to Brighton, Eastbourne and the rest of East Sussex.
The BBC understands that the incident took place in the Landport Road area of Lewes.
Network Rail has confirmed that no trains can run through between Haywards Heath and Lewes "until further notice".
Passengers travelling by rail should allow an extra 60 minutes if travelling in the affected area, Southern Rail says - while passengers travelling to Gatwick Airport are being asked to allow an extra 90 minutes.
Cooksbridge and Plumpton will not be served until further notice, they added.
A spokesperson for South East Coast Ambulance service said it was "currently responding to reports of an incident near Lewes Railway Station in Sussex".
They added: "We were called shortly before 16:00 BST and have a number of ambulance resources at the scene, including specialist teams, working alongside our emergency service partners.
"We have no further information to share at this time but will provide updates as soon as possible."
Lewes MP James MacCleary has said in a social media post that the derailment is a "very serious and developing incident".
He added: "I am aware of very concerning reports of a train crash in the Lewes area this afternoon.
"I am in touch with the relevant authorities now and will provide further updates as soon as I have more information.
"This is clearly a very serious and developing incident, and my thoughts are with everyone affected.
In the meantime, if you are concerned about someone or need urgent assistance, please contact the emergency services."
Ben Motley
Live reporter
The BBC's Ben Motley is near the site of the derailment, here's what he told BBC Radio Sussex:
I'm about a kilometre from the scene of the derailment.
One carriage is on its side at the bottom of a small embankment, with two other carriages on their side as well.
I can see three helicopters that have landed in a bend in the river, between the railway and the river.
There are many, many ambulances and fire engines down below tending to it.
I can see people - presumably passengers - being brought under a bridge under the railway to some of the vehicles that are waiting.
Earlier this afternoon emergency services were seen arriving at Lewes Railway Station following the derailment.
BBC reporter Nathalie Edell took this photo from the scene.
Two police vehicles outside Lewes Railway Station
The train operator Southern Rail has confirmed that a train has derailed in the area between Haywards Heath and Lewes.
"Emergency services are making their way to the scene and we are working out the impact this will have on our network," it has posted on X, external.
The operator adds: "At this time, no trains can run between: Brighton and Lewes, Haywards Heath and Lewes, Seaford/Eastbourne and Lewes."
The train operator says people should delay their journeys until later on today or use an alternative route to travel.
The rail operator Southern Rail is posting updates to passengers on its social media platforms.
It has posted this advice on X: “If you are currently on-board a train which has been at a stand for some time outside of a station, please remain on the train and await further updates from our train crew. If you have any additional support needs, please get in touch with us directly on X and we'll be able to assist further.”
Transport Secretary Heidi Alexander has said on social media that she is "deeply concerned" of reports of the derailment, which took place at about 15:55 BST.
She added: "Grateful to emergency services who are on the scene supporting those impacted. We are working quickly with the rail industry and local partners to assist passengers."
Photos from the scene show a train on its side with emergency service vehicles surrounding it.
People can be seen standing on top of the train carriage.
Image source, Laura Arnold / XA train has derailed near Lewes, Network Rail and Greater Thameslink Railway say
Here's the full statement from the British Transport Police on the train derailment near Lewes:
"Officers are currently on scene close to Lewes railway station following reports of a derailed train at 3.54pm today (13 August).
"We’re responding alongside paramedics and the fire service and will provide more information as soon as we can."
Network Rail and Greater Thameslink Railway are reporting that a passenger train has derailed near Lewes shortly before 16:00 BST.
Emergency services including South East Coast Ambulance Service, East Sussex Fire and Rescue Service and British Transport Police are at the scene.
A spokesperson for Network Rail and Southern Rail said: "We are aware that a passenger train has derailed in the Lewes area of East Sussex.
"Emergency services are on site and we will update with more information when we can."
