A fault in the signalling system between Gatwick railway station and East Croydon has led to services being cancelled or delayed.
Southern Rail alerted customers that three out of six lines were blocked.
Network Rail tweeted that its staff were on site and working to the resolve the issue.
Train services running through Gatwick may be delayed by up to 15 minutes or revised, according to National Rail, who added that disruption is expected until midday.
Commuter Phin Pope wrote on social media that there were ‘massive delays’.
Passengers are advised to check their journey before travelling.