Commuters have been told not to travel from London Waterloo during the rush hour after a fire closed nine platforms.
The lineside blaze damaged cabling outside the station, meaning trains cannot use platforms 16-24.
Network Rail said “significant damage” had been caused to equipment, meaning trains will be delayed or cancelled.
Disruption is expected for the rest of the day while the Thursday morning rush hour may also be affected.
Network Rail said its engineers would be working through the night to fix the damage.
Waterloo is the busiest and largest railway station in the UK.
The platforms which are closed are normally used by trains serving Windsor, Reading, Hounslow, Richmond and Kingston.
However, services from other platforms are also being affected because trains have to be diverted or revised.
- Circular services via Hounslow, Richmond, Strawberry Hill and Kingston have been cancelled
- Trains between Waterloo and Windsor & Eton Riverside are diverted via Kingston
- Trains between Waterloo and Exeter/Salisbury are terminated and will restart from Basingstoke
Passengers were warned that services on other routes may also be subject to short-notice cancellations or delays.
In a joint statement, Network Rail and South Western Railway said commuters were “strongly advised to use alternative routes where possible and check their journeys before travelling at southwesternrailway.com for ticket acceptance and service details”.
Some passengers took to social media to express their frustration at the travel disruption.
One Twitter user described the situation as an “absolute shambles”, while others complained about being given the wrong or no information at all by train station staff.
A barbecue on a balcony could have caused a block of flats to be engulfed by fire, according to its builders.
Twenty flats with wooden balconies were destroyed and 10 more damaged in Sunday’s fire in Barking, east London.
Residents said concerns were raised with builder Bellway Homes about potential fire hazards.
A Bellway spokesman said they it was “relieved that the fire protection measures… ensured that occupants were safely evacuated”.
The London Fire Brigade said the cause of the fire was still under investigation.
Fire investigators and scientific advisers have been at the scene carrying out “extensive investigations”, the service added.
It said: “We understand that the blaze was contained to the external envelope of the eastern elevation of the building, with initial reports suggesting that it was caused by a barbecue on a resident’s balcony.”
The Metropolitan Police said 15 to 18 families were evacuated from their homes and taken to the Thames View Community Centre, about a mile from the scene.
Following an investigation by BBC Watchdog in May, Peter Mason, chair of the Barking Reach residents’ association, said he asked Bellway Homes to review the fire risk.
In an email seen by the BBC from the firm last month, Bellway said the construction method used on the development in Scotland examined by Watchdog was different and so the Barking homes in De Pass Gardens were not affected in the same way.
More than 100 firefighters tackled the Barking blaze, which spread over six floors, for more than two hours.
A Bellway spokesman said the fire was “a very serious issue” and said it was working with all parties to establish how it happened”.
He added: “Whilst we are continuing our investigations into this matter, we are supporting London Borough of Barking and Dagenham in securing alternative temporary accommodation for affected residents and are offering our support to help remediate the damaged apartments.”