A number of cars were crushed against the concrete ceiling when an underground carpark was deluged with water yesterday morning.
Some vehicles' roofs crumpled after polystyrene insulation under the carpark floor appeared to swell, forcing them upwards.
The floodwater was caused by a burst pipe and was several feet high in the newly built car park, which is below a residential block of flats in Crayford, south-east London.
Residents were reportedly warned that if any of the cars were removed too hastily it could result in a gas explosion.
One owner was told her car was written off as a result of the damage.
The flooding caused considerable travel disruption after filling the road near Townhall Square, and it left a number of homes without water.
Sadia Farzana Chitra, 42, left her black Ford Fusion in one of the affected parking spaces on Saturday, and returned to find it ruined.
Between 12 and 16 cars are said to have been damaged.
The mother of two said: "My car is crushed, the roof is gone, the ground has come up - it is a bad architectural design.
"I had another of my neighbours with me and her car was okay, but we were told we cannot move it because there is a possibility with the pressure of the car there could be a blast because of the gas inside, or something. It can blow up the whole building."
She added: "The problem is who to contact. Someone has said the car insurance might not cover it because it didn't happen from the driver him or herself. We are in really, really bad shape."
London Fire Brigade said it was called to reports of the flooded roads at 3.21am, after a 24 inch water pipe burst.
Footage of the main road showed a nearby retail park submerged in water.