A signalling fault has caused disruption between Cardiff and Newport with several rail services affected.
Transport for Wales and National Rail has said that a fault with the signalling system between Cardiff Central station and Newport means trains will run at a reduced speed.
Delays of up to 30 minutes have been reported and disruption is expected to last until at least 5pm on Thursday, June 22.
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Passengers are being advised to check their journey before they travel.
The stations are both on the main route from South Wales to London Paddington, and means passengers waiting to board at Bristol Parkway, Swindon and Reading could all face delays due to the issue.
The Trainline website states that the disruption is expected to continue until 5pm, however journeys from Cardiff until after 7pm have all been issued with the warning.
Currently, on the Trainline website, the 7.18pm train from Cardiff to London Paddington is expected to be the last one affected by the fault.
Transport for Wales trains were also affected on the weekend because so many staff are currently self-isolating. At least 35 journeys were cancelled due to the staff shortages with the company forced to apologise to customers.
Live updates below: