Two people are seriously injured, with 18 others hurt, after a train derailed near Lewes in East Sussex
About 150 people were on board the train when it derailed shortly before 16:00 BST - here's what you need to know
Rob Bradley, who was on the train with his six-year-old daughter Mabel, tells the BBC: "It was pandemonium... I could feel the carriage rocking, then it jumped"
Charlie Hobden, from Lewes, says he was dragged along the floor when the carriage he was travelling in tipped over
"We first thought we may have hit someone," passenger Shannon Jones tells us. "Then we heard someone shout 'call the emergency services, it's really bad.'"
An image from the scene shows people standing on top of an overturned carriage and what appears to be a buckled track
Network Rail says it's too early to say what caused the derailment
British Transport Police Chief Superintendent Dominique Ioannou is giving an update on the derailment.
She says a number of passengers were initially trapped but were all safely evacuated from the train.
She says while two people sustained serious injuries, a further 18 suffered less serious injuries.
British Transport Police's chief superintendent Dominique Ioannou and the ambulance service are giving an update now.
Watch it live here.
Aerial footage shows derailed train
We're expecting British Transport Police to give an update at 9pm.
Watch the statement live here.
National Rail says major disruption between Lewes and Haywards Heath will continue "until further notice".
Southern says it has requested rail replacement buses, although these have not yet been confirmed.
Passengers can use their tickets on a number of alternative routes, including:
Passengers are advised to check before travelling as disruption continues across the network.
Joe Coughlan
Live reporter
Image source, PA MediaA major incident has been declared following a train derailing between Haywards Heath and Lewes in East Sussex shortly before 16:00 BST today.
Emergency services have said about 150 people were on board the train, with two people sustaining serious injuries and a further nine with less serious injuries. All of the passengers have evacuated the train.
Photos from the site show emergency workers around the train on its side in a field next to the tracks, with one passenger saying it "felt like a car crash" and another saying the train derailed from carriage five.
Operator Southern Railway has told passengers "do not travel" in the affected area for the rest of the day, with the Rail Accident Investigation Branch (RAIB) saying it has deployed a team of inspectors to the site.
Rozina Sini
BBC News
Image source, Charlie HobdenPassengers stand on the side walls of the train, with the seats above their heads, after it tipped over
Charlie Hobden, 31, from Lewes, was also on the train and said it "felt like a sliding snake", adding: "We just went over and we were dragging along the floor.
"We tried to open the doors and then we saw people standing on top of the train and so we managed to get out on our own. Others waited and got on ladders to get out.
"Then the emergency serves helped us out and got we got assessed. I didn't have any injuries so I was allowed to go home."
British Transport Police has confirmed that a major incident has been declared following the incident on Thursday afternoon.
The force said a number of passengers became trapped on the train but they have now been "safely evacuated".
Assistant Chief Constable Ian Drummond-Smith said: "We've declared a major incident and a significant emergency service response is ongoing at the scene, but all passengers on the train have now been safely evacuated from the service.
"Our thoughts are of course with everyone affected, and I’d like to reassure the public that we’re supporting the Rail Accident Investigation Branch to establish exactly what's happened.
"We’ll provide further information as soon as we are able."
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Hastings and Rye MP Helena Dollimore has been speaking to BBC South East today this afternoon.
She said: "I think all of us rely on our local trains and take for granted that they run safely.
"It will be extremely distressing for people to hear that there has been an accident of this nature.
"All of us are hoping that the injuries are as limited as possible."
As a reminder, South East Coast Ambulance Service has confirmed that two patients have sustained serious injuries, while a further nine passengers suffered less serious injuries.
We can bring you the latest pictures of the train which derailed near Lewes in East Sussex. The train is seen on its side in a field next to the tracks, with emergency workers arriving to investigate.
Image source, Eddie MitchellDamaged carriages on their side as seen from above
Image source, Eddie MitchellThree rear carriages came off the tracks
Rozina Sini
BBC News
More eye witnesses are sharing their accounts of the derailment with BBC Your Voice.
Shannon Jones, who was onboard, said the derailment "felt like a car crash", adding: "We all flew forward. The train started wobbling from side to side, it felt like the driver had slammed brakes.
"I was in the fifth carriage, so not the one that derailed. No one knew what was going on.
"We first thought we may have hit someone. Then we heard someone shout 'call the emergency services, it's really bad'.
"We heard the sirens and helicopters and a police officer motioned that help was coming and they needed to check the rail track wasn’t live.
"Then we had to go down a ladder one by one and then across the track and the embankment down the field."
Image source, Shannon JonesSouth 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."
It added that it continues to respond to the major incident alongside other emergency services and urged people to avoid the area.
We have heard from South East Coast Ambulance, who have confirmed that "two people sustained serious injuries".
Eye witness John May has shared photos of the derailed train showing passengers clambering out of the tipped over carriage.
Others are standing on top of the carriage as they escape from the scene.
Image source, John MayPeople were stood on top of the tipped over carriages after the derailment
Image source, John MayPeople were sat next to the side of the track after the derailment having escaped the carriage
Image source, John MayA number of the carriages which derailed tumbled into a field next to the railway
Image source, John MayNew pictures that have been shown to the BBC show people leaving the train, with carriages on their side after they left the tracks.
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 AkinFire crews and ambulance service are at the scene of the derailment
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."
John May, from St Leonards, was on the train when it derailed near Lewes
University Hospitals Sussex NHS Foundation Trust has confirmed that it is currently responding "to a major incident" alongside other emergency services.