The disruption, affecting Thameslink and Great Northern routes, means that some trains may be delayed or cancelled, throughout the day on Monday, January 19.
Overrunning engineering work between Welwyn Garden City and Alexandra Palace has blocked parts of the line and cut the number of trains that can run.
Thameslink and Great Northern said: “We strongly recommend you delay your journey until later today.
“Morning peak services are extremely busy, and you may have to wait longer for a train, or be left behind if the service is full.
“Also, you may need to change more, possibly extending your journey by at least 30 minutes.”
Services between Stevenage and London have been particularly badly affected, with delays of up to 20 minutes expected to continue throughout the day.
The disruption has resulted in the suspension of several key routes, including all trains between Welwyn Garden City and Moorgate, and Thameslink services between Cambridge and Brighton.
Some Cambridge to London King’s Cross trains are stopping at extra stations between Welwyn Garden City and King’s Cross.
Trains meant to run between Welwyn Garden City and Sevenoaks are only running between London Blackfriars and Sevenoaks.
Stations on the affected line, including Hatfield, Welham Green, Brookmans Park, Potters Bar, Hadley Wood, New Barnet, Oakleigh Park and New Southgate, have just one train an hour.
Trains stopping at these stations are expected to be packed, and some passengers may not be able to board.
Passengers may also need to change trains, adding more time to journeys.
Passengers are advised to check journey planners, station information boards, and live departure boards before travelling, and to allow extra time to reach their destinations.
Great Northern and Thameslink tickets are being accepted at no extra cost on other routes, including:
Tickets are being accepted on other reasonable routes: Thameslink, Great Northern and Southern; Greater Anglia between Cambridge and London Liverpool Street; LNER between Peterborough and London King’s Cross; London Buses between Potters Bar and London; the Tube across London including to Liverpool Street; and Uno buses 242 and 243 serving Hatfield, Welham Green, Brookmans Park and Potters Bar (plus Cuffley/High Barnet).
Passengers can take Centrebus route 305 between Brookmans Park and St Albans City at their own expense.
From St Albans City, Thameslink services to and from London are available.
LNER ticket holders can use Avanti West Coast between Manchester Piccadilly and London Euston, TransPennine Express between Manchester and Leeds or York, Northern between Sheffield, Doncaster, Leeds and York, East Midlands Railway between Sheffield and London St Pancras International, CrossCountry between Sheffield, Doncaster and York, and Greater Anglia between Peterborough and London Liverpool Street.
LNER tickets are also valid on Great Northern and Thameslink trains between Peterborough, Stevenage and London King’s Cross.
LNER passengers can travel at no extra cost on other services up to two hours before or after the cancelled train, or on either of the two LNER services before or after it.
Hull Trains and Lumo are largely unaffected, but passengers should still expect delays of about 10 minutes.
Passengers delayed may be able to claim compensation.
Thameslink advises keeping tickets and making note of journeys for any claims.
Passengers are encouraged to check the National Rail Enquiries Journey Planner for real-time updates before setting out.
