A council worker caused an ammonia leak at Barnet House this week when she reversed her car into three canisters of the hazardous chemical left in the basement car park.

An internal investigation has been called by the council after it admitted the 20-litre drums of liquid ammonia should not have been left out of secure storage at the borough's offices in High Road, Whetstone.

Five fire engines and a special decontamination van rushed to the scene shortly before 10am on Tuesday when the liquid ammonia leaked over the floor.

Firefighters used protective clothing and breathing equipment to decontaminate the drums and flush out the car park with water while employees were told they could not enter or leave their offices. The car park re-opened at 2pm.

The Health and Safety Executive (HSE) will also be examining why the ammonia, which can cause serious burns, was left unattended.

A spokesman for the council said: "We have launched an immediate investigation into the circumstances of this incident.

"These canisters were not left in a public area but placed outside a storeroom prior to collection by council contractors. They were against a wall away from the parking area designated for staff but unfortunately an employee accidentally drove into the canisters."

According to HSE regulations appropriate controls must be put in place to prevent employees from being exposed to hazardous substances wherever possible.