About 40,000 people in Southern California have been ordered to evacuate their homes as emergency responders race to prevent a large tank containing hazardous chemicals from leaking or exploding.

Residents in six Orange County cities were told to leave their homes on Friday after a storage tank containing up to 7,000 gallons (26,500 litres) of methyl methacrylate – a highly flammable chemical used in the production of plastics – began to fail the previous evening.

The tank is one of three located at GKN Aerospace in Garden Grove, about 49km (30 miles) from Los Angeles. GKN Aerospace manufactures parts for several aviation companies, including Airbus and Boeing.

Orange County Fire Authority Division Chief Craig Covey said the tank could either rupture, causing its contents to spill onto the ground and create a plume, or explode.

“That’s the best-case scenario, believe it or not, because once it comes out, it’s no longer an explosive hazard and our hazmat teams in their chemical protective suits can go in and neutralise and mitigate the vapours that will be coming off of that, the plume that will be coming off of that leak,” Covey said.

“The other option that was told to us is that it blows up. That is what we were handed: a leaking tank, or a tank that blows up.”

Firefighters were able to reduce the tank’s temperature from a safe distance, buying crews more time to stabilise the situation.

Garden Grove Police Chief Amir El-Farra said more than 4,000 people had refused to evacuate. Local mayor Stephanie Klopfenstein urged those still in the area to leave immediately.

“We understand it is frightening people who are worried about their homes, their businesses, their pets and loved ones. But this is a serious situation, and now is not the time to wait,” she said.

More than a dozen schools have also been closed, with officials warning they don’t know how long the evacuation order will remain in place.