Four companies competing to build a $12 million electronic toll collection system on the Dulles Toll Road are accusing the Virginia Department of Transportation of favoring a fifth firm to get the contract.

The companies allege that bidding specifications were written in such a way as to prohibit all but that firm from winning the contract.

The firm, Vapor Canada Inc. of Montreal, has business ties to the consultant that the state hired to advise it on the project, Castle Rock Consultants, whose local office is in Leesburg. The dispute was first reported in yesterday's suburban Journal newspapers.

By the middle of next year, state officials hope to have a system that will cut down on toll cheating and speed the flow of traffic along the 15-mile road between Tysons Corner and Dulles International Airport.

Under the system, drivers could go through the toll collection stations without stopping. Instead, a device attached to the car would be "read" by an antenna in the pavement. The toll would be subtracted from a prepaid account.

The four companies say that the specifications for the bidding process largely match the description of Vapor's electronic system.

Amtech Systems Corp. of Dallas makes a system that is read by an antenna on the side of the road or overhead.

"If we have a viable system, why is our technology excluded?" asked William H. Allen Jr., an Amtech vice president.

"If one technology is superior to another, that happens," responded Frank Lidd, secretary of Vapor's parent company, Vapor Corp. of Chicago.

Vapor and Castle Rock officials denied any impropriety. The companies have done business together in the past. The Dulles system is patterned after one being built by Vapor in several western states, with Castle Rock as a consultant.

The transportation agency's Aubrey C. Baird said yesterday that the specifications allow bids from more than one firm. He said, however, the deadline for bids would be extended to Aug. 30 to allow companies more time.

The other companies considering bids are IBM Corp., Westinghouse Electric Corp. and Gulf Systems Inc. of Baton Rouge, La.