Two city-bound lanes of the Hume Motorway remain closed after the trailer of a Woolworths truck caught fire early on Tuesday morning.

Police are warning motorists to expect lengthy traffic delays and preferably avoid the area after the inferno closed all four north-bound lanes of the motorway at St Andrews, near the Campbelltown Road exit.

Two lanes have since reopened, but traffic remains very heavy with 7km queues.

It's understood the driver of the truck managed to escape without injury.

More to come