First Capital Connect blames power problems for delays
A third of trains have been cancelled and others are subject to delays in Bedfordshire and Hertfordshire.
Commuters are facing problems on Thameslink lines between Bedford and London because of overhead wire damage.
The whole network is being disrupted because of power problems in the West Hampstead area of north London.
First Capital Connect said that only three out of the usual four rail lines into London stations are available and apologised for the cancellations.
Larry Hayman, from First Capital Connect, said: "It's happened at a very embarrassing time for us with the minister of state for transport opening a £20m car park at Luton station.
"We've had to cancel about a third of services on Wednesday because one of the lines through a tunnel near West Hampstead station has had to be closed.
"Overhead power cables came down and engineers are working to repair them."
Services are likely to be disrupted all day with cancellations and delays.