Emergency services outside the BAE Systems Glascoed plant in Monmouthshire, South Wales
The blast took place at the BAE Systems munitions factory in Glascoed, Monmouthshire shortly before 10.50am.
MailOnline understands the explosion relates to a single shell going off at the munitions filling factory, run by BAE, Europe's biggest military contractor.
Comment: One must bear in mind that the claim 'a single shell going off' cannot be verified.
Emergency services were deployed to the site, which is spread across a large field close to the south-eastern village; locals say the blast 'wasn't loud, but felt like an earthquake'.
BAE Systems has promised a full investigation into the incident at the factory, which is the UK's last remaining munitions filling factory.
BAE says the Glascoed plant employs around 500 people. The firm is Europe's biggest military contractor with several sites across the UK.
A local told the South Wales Argus: 'It wasn't that loud from where I was but it felt like an earthquake. My car shook.'
A BAE Systems spokesperson said: 'We can confirm that there was an incident at our Glascoed facility in Wales this morning.
'Tried and tested safety protocols were immediately enacted and emergency services attended the site.
'All personnel are accounted for and our emergency response team continues to monitor the facility. We have launched a full investigation.'
Sources have indicated that a shell exploded on part of the site, which is spread out across a large area around a square kilometre in size.
This is understood to be a safety measure to reduce the risk of casualties in the event of incidents involving munitions.
The source added: 'All the accounts we have so far are that a shell exploded - there's nothing to suggest anyone is injured and we have accounted for all personnel. The site is safe.'
A Welsh Ambulance Service spokesperson said: 'We were called shortly before 11:00am today to reports of an incident in Glascoed, Usk.
'We sent three operational managers, one Cymru high acuity response unit paramedic, one emergency ambulance and our Hazardous Area Response Team to the scene.
'No further treatment was required and we were subsequently stood down.'
Glascoed was established in the 1930s as the government began preparing for war in Europe - which later escalated into the Second World War.
BAE took over the plant from the British government in the 1980s after the country's Royal Ordnance Factories were privatised.
Glascoed is now the UK's last remaining ammunition filling facility in the country - meaning it is the last place munitions will go to have shells and other cases filled with explosive materials.
According to the firm's website, the site produces ammunition for small arms and medium-calibre weapons, as well as mortar bombs, tank ammo and artillery shells.
Comment: This doesn't bode well for the establishment's proxy war in Ukraine, as well as their brewing Middle East conflict, because they were already suffering from a shortage of various munitions: Russian arms production has Europe's warmongers worried
In recent years, it has been responsible for manufacturing the vast majority of munitions used by the UK armed forces in training and on the frontline.
These efforts include the manufacture of munitions as part of the UK's commitment to supporting Ukraine in its fight back against Russian invaders.
But the site has also been targeted by pro-Palestinian protesters who accused BAE of supplying jet fighters and munitions to the Israel Defense Forces.
In 2020, BAE Systems was awarded a new £2.4billion 15-year contract to supply munitions to the Ministry of Defence, securing 1,260 jobs across its five UK sites.
Gwent Police declined to comment on its role in the operation, directing MailOnline to BAE Systems' statement.
The Daily Mail, of all shabby British tabloids ... I wonder why they don't say it ?