A train has derailed near Lewes railway station in East Sussex.
Emergency services were called to the scene at around 3.55pm, with the public being warned to avoid the area.
According to reports, passengers are trapped in overturned carriages, with more than 40 believed to be on board.
It is understood three carriages of the eight-carriage train have overturned.
Images on social media, apparently from the scene of the crash, show people standing on carriages lying on their side in a countryside area.
Sign up for all of the latest stories
Start your day informed with Metro’s News Updates newsletter or get Breaking News alerts the moment it happens.
Southern Rail said the derailment had happened in the stretch of track between Lewes and the town of Haywards Heath further north.
‘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.
‘At this time, no trains can run between Brighton and Lewes, Haywards Heath and Lewes, and Seaford/Eastbourne and Lewes.’
A statement from East Sussex Fire and Rescue Service said: ‘We can confirm we were called at 3.55pm to reports that a train had derailed just outside Lewes.
‘We are working with other emergency services at the scene.’
Writing on X, Transport Secretary Heidi Alexander said she was ‘deeply concerned’ by the reports of a derailment.
She penned: ‘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.’
Meanwhile, James MacCleary, the MP for Lewes, wrote on Facebook that it is ‘clearly a very serious and developing incident’.
The general secretary of a transport trade union added that, ‘with so many passengers reportedly involved’ in the derailment, ‘the immediate priority must be their safety and wellbeing’.
A British Transport Police spokesperson said: ‘Officers are currently on scene close to Lewes railway station following reports of a derailed train at 3.54pm today.
‘We’re responding alongside paramedics and the fire service and will provide more information as soon as we can.’
Southern Rail has confirmed no trains can run between Lewes and Brighton, Haywards Heath, Seaford or Eastbourne as a result of the incident.
The derailment is causing major delays to the network south of London.
Got a story? Get in touch with our news team by emailing us at webnews@metro.co.uk. Or you can submit your videos and pictures here.
For more stories like this, check our news page.
Follow Metro.co.uk on Twitter and Facebook for the latest news updates. You can now also get Metro.co.uk articles sent straight to your device. Sign up for our daily push alerts here.