A message posted on the South Western Railway website said: “We have received reports that a person has been struck by a train between Wimbledon and Vauxhall.
“This incident did not involve one of our trains however our response teams are working with the emergency services to assist with the incident.
“This has caused significant congestion around Clapham Junction and may result in short notice changes to train services.
“To minimise the impact of this incident, suburban trains that use platform 11 at Clapham Junction will not stop at this station.”
This incident is also affecting trains running through Victoria and East Croydon and “services are at a stand” in these areas.
The delays are expected to last until at least 2pm.