Sluggish unloading at supermarket distribution centres is costing small growers a fortune, they claim. Abuse of the just-in-time delivery system regularly keeps trucks waiting for hours at suppliers' expense - even though growers face stiff penalties if they miss the 30-minute windows given to them by the supermarkets.

One grower said he lost £600 a week over the summer from delays. "It's pretty common practice a fair amount of the time for Tesco to insist on a loading time in a half-hour slot, then keep the truck waiting for one, two, five hours," he said. "It costs us a fortune to allow them to use our trucks as a cold store." The problem is likely to become more serious as stores expand their local sourcing initiatives, in which more growers and packers supply direct.

The delays keep the truck tied up for the best part of a day and mean the driver has to be paid for the extra hours doing nothing, the supplier said. With his two trucks, delays ran to an average of 24 hours a week, costing more than £12,000 across the season, he added.

Tesco's Chepstow depot was singled out as a serial offender, but Asda and Morrisons were also mentioned and 12-hour delays were not unknown. "They can unload it in 20 minutes if it suits them, but the depots are so unionised they're very slow," said another grower.

A Midlands-based haulier specialising in chilled food said he no longer accepts nine out of 10 jobs to supermarket depots. "I wouldn't want to be going into any retail depot this month with the Christmas rush."

He said he had been kept waiting 12 hours at Tesco's Middlewich depot with direct costs of at least £25/hour. "Some retailers tip their own lorries off first."

Tesco said the timed delivery slots had been generally welcomed by suppliers. "It is in no one's interests for drivers to wait, and we will work hard to ensure this does not happen," said a spokesman.