Two passenger trains have collided in the Bedford area, leaving a number of passengers and crew hurt.
Emergency crews are on the scene after the crash happened between two East Midlands Railway (EMR) services at 17.12 BST.
Transport Secretary Heidi Alexander said she was "deeply concerned" by reports of the collision, while the Rail, Maritime and Transport (RMT) union said passengers and staff had sustained "serious injuries".
Bedfordshire Fire and Rescue crews are attending the site just south of Bedford and the service asked people to avoid the location while they worked at the scene.
British Transport Police announced just before 18:45 BST it was responding to reports of a collision near Bedford.
Rail service Thameslink reported lines between Luton and Bedford had been blocked since around 17:30 BST.
A passenger who was on board one of the trains, who spoke to the BBC, said the collision happened at 17:12 BST.
The collision happened just south of the Elstow interchange between the A421 and the A6 and both trains are still on the track.
Family and friends of passengers have been told not to go the scene by police.
It has been claimed one of the trains hit another that was either stationary or moving slowly in front of it on the same line.
Passenger Peter Knapp said others on his train saw one ahead of them before the crash.
He told BBC News: "I felt like I'd been in a bomb explosion."
Knapp described seeing "bloodied faces", people with what appeared to be broken legs, and "smoke everywhere".
Another passenger told the BBC that people flew out of their seats in the crash, including someone who hit her husband in the face.
"There was a lot of blood. A lot of people had facial injuries," Shola Mene said.
The RMT union said it had received reports that passengers and onboard train staff have sustained what it described as "serious injuries".
The Rail Accident Investigation branch said it was in Bedford trying to find out exactly what had happened.
It said in a statement on X: "A team of RAIB inspectors is on site at the scene of a collision between two trains near Elstow, to start gathering evidence."
East Midlands Railway 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.
"We are currently on the scene and working with Network Rail to assist the emergency services.
"Further updates will be provided as soon as more information is established."
The train company had earlier posted online that services to and from London St Pancras were suspended for the rest of the day.
British Transport Police says it is responding to a collision between two trains in the Bedford area.
"We understand that people are concerned about relatives or friends who may have been travelling on the trains involved in the incident near Bedford," a spokesperson added.
"We're asking people not to travel to the scene and await further updates from emergency services."
Bedfordshire Fire and Rescue Service said: "Crews are currently in attendance at an incident on the railway just south of Bedford.
"Please avoid the area. Thank you for your co-operation."
East of England Ambulance Service said: "We have sent a number of resources, including air ambulance and our Hazardous Area Response Team, to a major incident on the railway south of #Bedford. We encourage people to avoid the area.
"Further updates will be provided in due course. Please only call 999 in an emergency."
Transport Secretary Heidi Alexander said she was worried by reports of the collision.
She said on X: "I'm deeply concerned to hear reports of the collision involving two East Midlands Railway passenger trains.
"I'm grateful to emergency services who are on the scene, attending to those affected.
"We're working quickly with the rail industry and local partners to support passengers."
Bedford and Kempston MP Mohammad Yasin said the "very serious" incident was "really concerning" and told the BBC he believed there was "a number of casualties".
Health Secretary James Murray said: "I am being kept updated on the collision of two trains between Luton and Bedford.
"A number of people have been injured and I thank first responders who are helping those affected."
