Two passenger trains have collided in the Bedford area, leaving a driver dead and dozens of passengers and crew injured.
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, the British Transport Police (BTP) said a major incident had been declared.
Nine people were in a critical condition as of Saturday morning, out of a total of 28 who were still in hospital, according to the emergency services.
The total number of people who were seriously injured in the fatal train collision on Friday has been raised to 32 in an update by the East of England Ambulance Service.
The number of people who suffered minor injuries has also increased to 57.
This brings the total number of people injured as a result of the incident to 100, including the 11 individuals who sustained "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 around 17:12.
Rail service Thameslink reported that lines between Luton and Bedford had been blocked from around 17:30.
The BTP said just before 18:45 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 declared a major incident shortly after the crash.
Chief Constable Lucy D'Orsi said more than 80 people had been treated in hospital following the collision.
The East of England Ambulance Service (EEAS) said 11 people had sustained very serious injuries and 22 had been seriously injured.
It took a total of 64 people to hospital, while some people with minor injuries were treated at the scene.
Paul Gates, EEAS strategic commander, said 20 ambulances and six air ambulance "partners" were dispatched to the crash site, among other resources.
Bedford Hospital and Luton and Dunstable University Hospital asked people to avoid attending their emergency departments "unless they have a genuine medical emergency" while they treated victims of the crash.
Bedfordshire Fire and Rescue said that, while the crash did not cause a fire, more than 70 firefighters had attended to help with the emergency response.
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 (RAIB) 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.
The BTP's Chief Constable D'Orsi stressed that the force's "extremely experienced" specialist investigators were working with the RAIB "to gather the facts and determine what has happened".
She added: "I would ask that we all refrain from speculation."
