A freight train which derailed and caught fire in South Wales consisted of 25 wagons each holding up to 76 tonnes of diesel or gas oil, investigators have said.

The accident happened as the train passed through Llangennech, near Llanelli, at 11.15pm on Wednesday.

The Rail Accident Investigation Branch (RAIB) said the derailment and subsequent damage to the wagons “resulted in a significant spillage of fuel and a major fire”.

Local people reported “seeing a plume of flames and smoke, and the strong smell of fuel”, the RAIB added.

Ten of the wagons derailed.

The driver, who was unhurt, reported the accident to a railway signaller.

The RAIB said its investigation will seek to identify the causes of the derailment and how these led to the fire, as well as “any underlying management factors”.

Mid and West Wales Fire and Rescue Service said the fire has been extinguished and control of the scene was handed over to British Transport Police (BTP) and Network Rail at 8.30am on Friday, more than 33 hours after the accident happened.

There were fears that diesel spillage into the nearby River Loughor could spell disaster for the local cockle fishery industry, with the Food Standards Agency advising the closure of shellfish beds in the area’s estuary.

The train, owned by DB Cargo UK, was travelling from Robeston oil refinery in Milford Haven to a fuel distribution terminal in Theale, Berkshire.

Witnesses described hearing a loud bang before seeing the fire.

Officers declared a major incident and knocked on the doors of houses in an 800-metre exclusion zone, asking around 300 people to move to the nearby Bryn School and Llangennech Community Centre.

People were allowed to return to their homes after 5am on Thursday.

Inspector Mike Edwards, of BTP, said: “I can reassure those in the local community that we have worked hard with partners to make the scene of the incident safe.

“This has enabled colleagues at Natural Resources Wales and their specialist teams to fully assess the environmental impact and advise DB Cargo and Network Rail when the clean-up of the site can begin.”