Movers who add illegal weight to truckloads of household goods shipments are costing American consumers $20 million a year, according to Interstate Commerce Commission chairman Dan O'Neal.

He announced a campaign yesterday to combat what he termed "a flagrant and despicable consumer rip-off" on the part of some movers.

In approximately 9 per cent of the 1 million moves made annually in this country, the commission found that movers had illegally added extra weight in the form of bricks and iron bars or had falsified weighing tickets to jack up prices. In the cases studied, the average overcharge was more than $500 for each shipment.

At a press conference, O'Neal presented a film made by undercover agents in Santa Clara County, Calif. Movers were shown adding crates of bricks and ignots - and occasionally people - to shipments that were to be weighed. After weighing, the extra weight was removed. Between 300 and 4,300 pounds was added on six different loads, with overcharges ranging up to $935 a load.

As a result of the film, an independent agent for King Van Lines and Republic Van & Storage Co., Carl Culpepper, was arrested and convicted of grand theft and fraud. (The companies immediately severed their relations with him.)

In California - as in many other states - there are relatively stiff civil and criminal penalites for weight "bumping," as it is called. O'Neal pledged to enforce current ICC regulations that call for a maximum fine of $500, but no imprisonment. He also said the commission was looking toward the new legislation to strengthen penalties and to make consumer claims for losses easier.

O'Neal was asked how consumers can avoid being cheated. In a film, a woman accompanied her shipment to the weighing station, but this did not stop her agent from putting the ingots in the van beforehand. O'Neal admitted there was no foolproof way of stopping the practice.

The commissioner also announced an "all-out offensive" against illegal agricultural cooperatives which, he said, are siphoning off some $350 million in business annually from commom carriers. Unlike common carriers, which must file rates with the ICC, co-ops are not regulated in order to enable farmers to get their foodstuffs to market more cheaply.

However, hundreds of sham co-ops - which have no farmers as members and do not haul produce - have sprung up recently. These haul machinery and other cargo at rates well under those set by ICC.

In related news, the ICC has released its 1976 performance data records on the 20 largest movers. The company having the least number of shipments that were billed at more than 10 per cent above the estimated price was Wheaton Van Lines. The worst record, as reported by the companies themselves, belonged to Burnham Van Service.

The company having the least delay in picking up goods was Imperial Van Lines; the worst, King Van Lines. The worst offender for the late deliveries was National Van Lines. And the company with the highest percentage of claims over $50 was Aero-Mayflower Transit.

On the basis of these four categories, the following companies proved above the mean average: Allied, Global, Ivory, King, Lyon, North American, Pan American, TransAmerican, United and Wheaton. Those below: Aero-Mayflower,American Red Ball, Atlas, Bekins, Burnham, Imperial, National, Pyramid, Republic, and Smyth.