An investigation has been launched into an explosion at an industrial complex in Rettendon, Essex which led to 20 firefighters being taken to hospital.

The blast, which was triggered by flames spreading to a fertiliser store, is being investigated by the Health and Safety Executive (HSE). Over 100 firefighters were called to the scene at Highlands Farm, where the fire had been spreading through business units.

HSE inspectors will examine the scene of the explosion, which was compared to a "sonic boom" due to the noise it made, as well as speak to witnesses and work with Essex County Fire & Rescue during their inquiries.

Of those taken to be treated at hospital, four suffered minor injuries and the rest were treated for issues such as loss of hearing.

The HSE also confirmed that Essex Police would be a part of the investigation. 

A spokesperson said: "The focus of HSE's investigation is to establish whether there has been any breach of workplace health and safety regulations. Our inspectors continue to work in close co-operation with Essex County Fire & Rescue Service as its firefighters work to bring the site to a safe resolution."

The explosion caused £1m worth of damage in lost equipment to a local family-run metalworks.