Refurbished South Western Railway trains withdrawn
Newly-refurbished trains have been withdrawn from service over fears they wrongly turn signals yellow or red.
The 30-year-old Class 442 trains were reintroduced by South Western Railway (SWR) in June after a £45m upgrade.
The firm said they had been taken out of service as a precaution due to an issue involving line-side signals.
SWR said it would cancel two daily services and shorten some trains until the issue was resolved.
The mothballed trains - nicknamed "plastic pigs" - have been running on routes from London to Bournemouth and Portsmouth.
They are suspected of accidentally turning signals yellow or red as they pass through Earlsfield in London, BBC Transport Correspondent Paul Clifton said.
He added: "While the fault is checked, on this very busy line a whole queue of trains builds up behind, delaying passengers."
SWR said it was working with Network Rail, which oversees signals, to correct the issue.
It said a morning service from Southampton Airport Parkway to Waterloo and an afternoon service from Waterloo to Poole would be cancelled as a result.
Shorter trains will run on five daily services between London and Farnham, Guildford, Haslemere and Woking, the firm added.
In a statement, SWR said: "We appreciate how inconvenient this will be to passengers on these trains and are sorry for any inconvenience caused."
Class 442s were originally due to be brought back into service in December 2018 after refurbishment in Bournemouth and Eastleigh.
The delayed reintroduction was aborted in May due to a safety problem with the door locks.
The first trains came into service in June, but new motors to improve reliability have yet to be fitted.