Three workers who were injured by a suspected gas explosion at a Mugg & Bean restaurant in a mall in Kempton Park, east of Johannesburg, remain in hospital.

Two of the three staff members from the restaurant were expected to be discharged from the Arwyp Medical Centre's ICU soon, but one of them was still in a critical condition.

Mugg & Bean manager Graeme Morrison said seven workers were injured when the suspected gas explosion rocked the restaurant in the Festival Mall in Kempton Park on the East Rand.

Several other people who tried to assist them were also injured. The explosion took place at about 8am.

"We are grateful that few staff were critically injured and [we] are holding thumbs for the three in critical condition in hospital," said Morrison.

Morrison said it was still not clear what caused the explosion.

"We presume it was gas. What we are finding hard to piece together is how it happened as equipment perishes over the years and this was a new, up-to-date installation," said Morrison.

The liquid petroleum gas that Mugg & Bean was using is stored in a main storage tank at the mall and piped to all the food outlets in the centre. The gas is supplied by gas company Afrox.

Afrox spokesman Johann Cilliers said that the problem "happened specifically in the restaurant".

Engineers who visited the site determined that there was no risk to other food outlets in the mall using the same gas, he said.

Cilliers said Afrox, the LP Gas Safety Association and the fire department were working together to determine the cause of the blast.

Festival Mall marketing manager Lee-Anne Leathley said the centre and area around Mugg & Bean remained "structurally sound" and customers didn't need to worry" about eating at the mall's restaurants or shopping there.

LP Gas Safety Association's spokesman Kevin Robertson said gas explosions are very rare as gas is an "inherently safe product and is used extensively across the world".

"No foul play is suspected," said Ekhurhuleni's emergency services spokesman William Ntladi.