Ms Jenkins said: "I was told that if they rose up, they would be able to take over in less than 10 minutes, and that this was close to actually happening recently.

"The staff member implied that they were very short staffed also and so they are worried that they would not be able to react sufficiently in an emergency situation."

A South Wales Police spokesman said they were investigating after a "bottle of unknown liquid, erupted and slid across the floor." He added: "It was a minor incident."

The Ministry of Justice confirmed no prisoners or staff were hurt in the incident on June 15.

A spokesman said: "The matter was referred to the police and it would be inappropriate to comment further at this stage."

Dr Ben Ward, from Cardiff University, told how coffee and tea whiteners can be turned into explosives if set alight.

He said: "Any very finely divided organic powder like coffee and tea whiteners is combustible and can really burn quite quickly.

"The smaller the particles the more likely they are to burn and if they burn quickly enough they can cause an explosion.

"It's the same with tiny grains of flour, custard powder and coal dust which was a major cause of explosions in coal mines."