The Justice Department filed a lawsuit against the U.S. Postal Service yesterday in an attempt to prevent it from starting its electronic mail system Monday.

The new system, called E-COM, is one of the postal service's highest priority projects designed to propel it into the electronic age.

The Justice Department said in its suit filed in U.S. District Court for the District of Columbia that the postal service violated the Postal Reorganization Act of 1970 by failing to seek a hearing by the independent Postal Rate Commission before starting the service.

The suit is the latest of several attempts over the past three years to to stop E-COM. The Justice and Commerce departments as well as private telecommunications firms have said the service unfairly prohibits private industry from entering the electronic message-delivery business.

The Postal Service said in a three-sentence statement that it is aware of the suit and that its lawyers are studying a response. "Barring a court decision to the contrary, E-COM service will begin on Monday, Jan. 4, as scheduled," said Postmaster General William F. Bolger.

The rate commission earlier this year recommended an experimental electronic mail service, but the U.S. Court of Appeals for the District of Columbia said the commission didn't have the authority to establish an experimental service. The rate commission then began expedited hearings on whether E-COM should be permanent, but postal service officials refused to participate, saying the commission didn't have the authority to determine the issue, the lawsuit said.

The hearings were suspended on Dec. 3, but the postal service decided to go ahead with E-COM anyway.

"The department has been a party in the commission's proceeding and filed the suit in order to protect the government's right to obtain a full public hearing before the Postal Rate Commission on whether the Postal Ser vice--a public tax-supported entity--should be engaged in competition with private sector firms capable of providing E-COM-like services," the Justice Department said in a statement. "If the suit is successful, the Postal Service would be required to follow the administrative procedures set out in the Postal Act before E-COM is implemented."

A source close to the case said Justice officials were engaged in negotiations with the postal service up to the last minute but those efforts were fruitless. The Justice Department then decided to file its complaint.

The government will ask a federal judge to hear its request for a temporary restraining order and a preliminary injunction today, Justice officials said.

So far, more than 100 companies have signed up to use E-COM. They include Merrill Lynch & Co., Shell Oil Co. and Columbia Gas Systems Corp. Under the system, businesses would send messages from their computers to the postal service's computers. The messages are then supposed to be printed, stuffed in envelopes and delivered anywhere in the country in less than two days.

The system has received criticism from telecommunications companies that claim the postal service wants to compete with them. Postal officials have contended that they don't want to compete with the private sector but just improve mail service through electronics.