Network Rail is suing two rail companies for more than £5m after a fuel spill caused by a train derailment threatened an "environmental disaster" at a designated site for protection.

The body filed a legal claim earlier this month against DB Cargo UK and Touax Rail over the derailment near Llangennech, Carmarthenshire, in August 2020.

The incident saw 10 wagons of the 25-wagon train derail, causing 446,000 litres of fuel to spill, and about 300 people had to leave the area due to the blaze.

In High Court documents, Network Rail claimed that the derailment occurred because the wagon had "not been maintained with reasonable care" and is seeking damages for negligence.

No one was injured, but 116,000 litres of fuel ignited while 330,000 litres drained into surrounding wetlands, which form part of a Site of Special Scientific Interest (SSSI) and a Special Area of Conservation.

The Rail Accident Investigation Branch (RAIB) concluded in January 2022 that the derailment was caused by a set of wheels on one of the wagons locking, likely the result of a brake defect after "inadequate maintenance practices" were found in the investigation.

Network Rail, which owns, operates, maintains and develops Britain's railways, claimed that the derailment occurred because the wagon had "not been maintained with reasonable care" by Touax.

It is also seeking damages from DB Cargo UK, claiming that the latter "breached a contractual duty to ensure that reasonable care was taken" in maintaining the wagon.

Lawyers for the body said it expected to recover "more than £5m" with DB Cargo and Touax yet to file defences.

A hearing in the case is due to take place.

The locomotive was pulling the wagons from Milford Haven, Pembrokeshire, to Theale, near Reading, when the derailment occurred.

The line was closed for six months after the incident and did not fully reopen for almost a year, with 30,000 tonnes of fuel-soaked soil having to be excavated.

Bill Kelly, the then-Network Rail Wales route director, said when the line fully reopened in March 2021 that work following the incident was "one of the biggest environmental recovery operations Network Rail has ever been involved with".

He added that the derailment could have caused a "total environmental disaster".

The RAIB said, in its 2022 report, that the train was travelling at about 38mph at the time of the derailment, and was carrying up to 75.5 tonnes of fuel.

The RAIB did not establish blame or liability for the incident, but made nine recommendations to avoid similar accidents.

Network Rail and DB Cargo declined to comment, while Touax has been approached for comment.