A general view of St Albans City station
Railway trains are being cancelled or delayed as a trespasser is on the track.
Earlier all lines were blocked between West Hampstead and St Albans due to a trespasser.
This story has now been updated, as lines have reopened in the direction towards St Albans City / Luton, however trains are unable to run from St Albans City towards London.
This is causing much travel disruption, with the National Rail warning that services between London St Pancreas and St Albans will be interrupted.
West Hampstead, Cricklewood, Hendon, Mill Hill Broadway, Elstree & Borehamwood, Radlett and St Albans City were blocked.
Disruptions are currently estimated to continue until 3.30pm.
The National Rail said: “If you are travelling on this route, please expect to reach your destination up to 15 – 20 later than planned as you may need to use an alternative route.”
Tickets can be used with no extra cost on London buses between Elstree & Borehamwood, Mill Hill Broadway, West Hampsted and St Pancreas or the London Overground and East Midlands Railway.