Rail commuters on Thameslink train services are facing major disruption due to a damaged overhead wire, the operator has said.
Two of the four lines between London St Pancras and Bedford are closed.
Commuters were warned there would be “significant reductions” in trains during the morning rush hour and disruption was likely to last for the rest of the day.
Thameslink has advised customers to check before they travel.
Tickets were being accepted on other routes including the London Underground, it added.
However, Tube commuters on the Metropolitan and Jubilee lines have also been caught in major disruption.
A broken down train near Neasden meant both lines had to be closed between Wembley Park and central London during the rush hour commute.
Jenny Saunders from Thameslink said: “Overnight there was significant damage to the overhead lines that provides power to our trains just south of Luton.”
There will be a “full investigation” and extra staff are on site to help, she said.
“It’s not an acceptable service that we are delivering for our passengers this morning and it is imperative that Network Rail and Thameslink continue to work together over the coming weeks and months to understand what is happening when incidents like this cause disruption,” she added.
Harpenden commuter Linzi James said: “Currently on train rammed full of people, that’s not moving. Not a good start to the day.”
She added overhead lines were “constantly failing” and “the impact on people is enormous”.
At the scene
BBC London reporter Sarah Lee
Passengers are packed on to the platforms of St Albans City station.
With only two of the usual four lines open it could take a while for people to get into London.
Dozens of rush hour trains have already been cancelled and delayed leaving people waiting in the freezing cold.
When trains do arrive as many commuters as possible cram into whatever space there is in carriages.
A spokesperson for Network Rail said: “We apologies for any inconvenience this disruption may cause.
“We are working hard to resolve the issues as fast and as safely as possible.”
The Labour MP for Bedford, Mohammad Yasin, said the service was “no better” than Northern Rail which is being renationalised.
“I met the rail minister a couple of weeks ago and mentioned all this and urged him to look into this case urgently because the people of Bedford are suffering,” he said.