The Crown Prosecution Service says there is "insufficient evidence" to proceed with corporate manslaughter against companies following an explosion at the Chevron oil refinery in Rhoscrowther, Pembrokeshire in 2011 which killed four people.
The CPS says it has made the decision following a review of evidence.
Julie Jones, Dennis Riley, Robert Broome and Andrew Jenkins were all killed in the blast, with a fifth person, Andrew Philips, suffering severe burns.
Following the decision made by the CPS, the investigation will now continue by the Health and Safety Executive into whether there are any breaches of the Health and Safety at Work Act.
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