Two passenger trains have collided in the Bedford area, leaving a driver dead and dozens of passengers and crew hurt.
Emergency crews were on the scene after the crash happened between two East Midlands Railway (EMR) services shortly after 17:00 BST on Friday.
Confirming the death shortly before 21:00 BST, the British Transport Police (BTP) said a major incident had been declared.
The East of England Ambulance Service said the crash had left 89 people injured to varying degrees, with 11 of them suffering "very serious injuries".
Prime Minister Sir Keir Starmer said reports of the collision were "hugely concerning", and that his thoughts were with the family of the person who died and with those seriously injured.
The crash happened just south of the Elstow interchange between the A421 and the A6.
A passenger on board one of the trains told the BBC that the collision happened at 17:12 BST.
Rail service Thameslink reported that lines between Luton and Bedford had been blocked from around 17:30 BST.
The BTP said just before 18:45 BST that it was responding to reports of a collision near Bedford.
Passengers say one of the trains had been stationary when the second hit it.
Dr Peter Knapp told BBC News: "I felt like I'd been in a bomb explosion."
The National Union of Rail, Maritime and Transport Workers (RMT) confirmed that one of the train drivers had died as a result of the crash.
General secretary Eddie Dempsey said: "The thoughts of RMT are with their family, friends, colleagues and the Aslef trade union at this awful time."
EMR has said its services to and from London St Pancras have been suspended over the weekend, with trains beginning or ending their journeys at Bedford.
This means people planning on travelling directly to St Pancras from places like Sheffield, Lincoln and Derby will have to take alternative routes.
The train operator has advised travellers that tickets which had already been bought could be used at no extra cost to travel with other operators.
Network Rail's East Midlands route director Mark Budden said: "We know passengers will want to understand when services can resume.
"We will provide updates as soon as we have a clearer understanding of the recovery work required and when the railway can safely reopen."
The disruption means that there are no direct trains from the Midlands and parts of northern England to Luton Airport and St Pancras.
Travellers would usually be able to change trains for a Thameslink service at Bedford - but there is engineering works this weekend and a replacement bus service running between Bedford and St Pancras instead.
Luton Airport said it was aware of the rail incident and urged air travellers to check the latest transport information before setting out for the airport.
EMR has confirmed two of its trains were involved in the collision.
A spokesperson said: "The 16:40 EMR train from Corby to London St Pancras has been involved in a collision with 15:50 Nottingham to London St Pancras, at Bedford South.
They added that it would be unable to retrieve luggage left aboard the two trains that crashed until the BTP had completed its initial investigation.
The BTP said: "A major incident has been declared, and officers are continuing to respond at the scene alongside colleagues from Bedfordshire Police and the local Fire and Rescue and Ambulance Services."
The East of England Ambulance Service said in a statement that 11 people had been left with very serious injuries, 22 had been seriously injured and 56 people had minor injuries.
It said it had dispatched "a number of resources", including an air ambulance.
Bedford Hospital and Luton and Dunstable University Hospital have asked people to avoid attending their emergency departments "unless they have a genuine medical emergency" while they treated victims of the crash.
Calling the incident "hugely concerning", Prime Minister Sir Keir Starmer paid tribute to the emergency services for their swift response to the crash.
"First and foremost, my thoughts are with the family of the person who has sadly lost their life, and with those who have been seriously injured," he wrote on X.
Transport Secretary Heidi Alexander said she was "deeply saddened" by the death.
Bedford and Kempston MP Mohammad Yasin said the "very serious" incident was "really concerning".
Health Secretary James Murray said he was being kept updated on the emergency response to the crash.
"A number of people have been injured and I thank first responders who are helping those affected."
The exact cause of the crash - and why one train was stationary ahead of a moving train - remains unclear.
As well as the BTP investigating, Rail Accident Investigation Branch inspectors have attended the scene to begin gathering evidence and will conduct a full investigation.
Alexander said it was too early to speculate on what that investigation would conclude, while stressing the UK's railways were "some of the safest in the world".
"We will make sure that there's a thorough investigation done to establish how this collision happened and to ensure that lessons are learned so that we don't have an incident like this ever again," the transport secretary said.
