Passengers are facing major disruption at London Euston after lines were closed for more than two hours due to a trespasser.
The delays began at around 8am this morning when the power on the tracks was turned off so the trespasser could be removed.
The person was spotted close to track near Carpenders Park, south of Watford.
The line was reopened at about 10:25am, but passengers are still facing major delays.
Network Rail said delays would have a knock-on effect for the rest of the day.
It said routes serviced by London Northwestern Railway, London Overground, Southern and Virgin Trains
Train services running between Watford Junction and London Euston and Clapham Junction were majorly disrupted and may be cancelled or delayed by up to 80 minutes.
The London Overground between Watford Junction and South Hampstead and London Euston was also affected, as well as Southern services between Watford Junction and Wembley Central.
Virgin Trains to major hubs such as Birmingham New Street, Chester, Manchester Piccadilly and Glasgow Central were also being delayed or cancelled as a result of the disruption.
National Rail has advised passengers to check with train operators before travelling.
Several passengers tweeted that they were struggling to get home for Father's Day and that replacement services were overcrowded.
They also shared pictures showing the timetable screens with no departures listed.
