Thameslink trains have been suspended between Gatwick Airport and Purley after trespassers were reported on the tracks, with all lines blocked and major disruption declared.
The incident was reported on Thursday morning, and passengers travelling to, from and through Gatwick Airport are facing severe disruption.
Follow our live blog below for the latest updates as the situation develops.
What has happened?
Trespassers were reported on the railway between Gatwick Airport, in West Sussex, and Purley, in south London, on Thursday morning.
All lines have been blocked as a result, and services on the route have been suspended.
Major disruption has been declared to all routes to, from and through Gatwick Airport.
Which services are affected?
The suspension affects Thameslink services running between Gatwick Airport and Purley, on the Brighton Main Line into London.
The disruption also affects services run by Gatwick Express, Great Western Railway and Southern, as well as Thameslink.
Purley sits on the southern edge of the capital, in the London Borough of Croydon, and the line through it carries Thameslink trains between Gatwick Airport, East Croydon and central London.
With all lines blocked, no trains are able to run through the affected section while the incident is dealt with.
Advice for passengers
Passengers planning to travel to or from Gatwick Airport, or through Purley and East Croydon, should check live departure boards before setting off and allow plenty of extra time for their journeys.
Commuters travelling to or from Gatwick Airport have been warned that services are being disrupted and that they will need to use an alternative route to complete their journey while the trespass incident is ongoing.
Airport passengers in particular should check their travel plans carefully, as the railway is the main link between Gatwick and central London.
We will update this live blog as the situation develops, including any reopening of the lines and the latest on delays.
Why trespass stops the railway
When trespassers are reported on the tracks, train operators must stop services through the area while the railway is checked and the people involved are found and removed safely.
It is the same reason services were halted earlier this week when a person was hit by a train near Stevenage, closing the line between London and Stevenage for more than two hours on Tuesday afternoon, and when a person was hit by a train at Catford Bridge, suspending Southeastern services between Hayes and Ladywell.
Thameslink services were also suspended in November when a person was hit by a train between Peterborough and Huntingdon, closing the line for several hours.
In each case, services could only resume once the railway was confirmed safe to use again.
A busy commuter route
The Brighton Main Line through Purley is one of the busiest commuter routes into London, carrying Thameslink and Southern services between the south coast, Gatwick Airport and the capital.
With the morning rush hour under way, the suspension is likely to affect thousands of passengers travelling into London for work, as well as people heading to Gatwick Airport for flights.
Passengers are advised to check for updates before travelling and to allow extra time.
This is a developing story. Follow the live blog above for the latest updates.
