The annual winter air fare war over the Atlantic began yesterday, two months later than expected.

World Airways won approval for a new low fare from Baltimore to London and British Airways reapplied for cheaper fares that had been turned down this fall in an Anglo-U.S. battle over antitrust enforcement.

The British government cleared the way for the latest price battle with the announcement that it will lift its ban on the low-cost winter fares. The British action came after the U.S. Justice Department told British Airways Thursday that cutting winter fares would not be considered an antitrust violation.

"The U.S. government has on this occasion provided assurances on the basis of which, in the interest of the traveling public, we are able to approve low and competitive fares for the remainder of this winter," The Associated Press quoted Michael Spicer, Britain's undersecretary of transport, as saying.

World Airways was the first carrier to receive authorization from Britain's Civil Aviation Authority and the U.S. Civil Aeronautics Board for a new low-fare, $338 round trip from Baltimore-Washington International Airport to London.

The British Airway request for low winter rates included a New York-London round-trip fare of $378, a $201 savings over its present lowest fare.

It was unclear last night whether BA's major American competitors, Pan American World Airways and Trans World Airlines, would match its fares in view of strong off-season passenger traffic this winter. Both Pan Am and TWA, however, sold cut-rate tickets matching the BA fares until they were forced by the British government to stop in October.

In a London announcement, Britain's Civil Aviation Authority said it told airlines flying the New York-London route that applications filed before Thursday "will be approved for Jan. 1."

World Airways said the new round-trip rates would start Jan. 15 and run until March 31. They will be on flights to London from Baltimore, San Francisco/Oakland, Los Angeles and Kansas City.

The new BA round-trip fares from New York, also available until the end of March, would be $378 on week days and $428 on weekends. The fares starting from London would be less expensive -- the equivalent of $303 -- because of the falling value of the British pound and the strong American dollar.

All the low fares have certain restrictions.

The big-name airlines were ready to start the new winter fare structure in the fall. But the British government blocked the cut-rate fares as a weapon in its longstanding dispute with U.S. authoritities over the use of American antitrust laws on foreign airlines flying the transatlantic route.

To underscore how seriously it viewed the antitrust issue, the British government took the unusual step of declaring invalid about 130,000 cut-rate tickets, most held by Americans, that had been sold in anticipation that the the low fares would win approval. Britain later relented.

The Justice Department, spurred by a civil antitrust suit, started a grand jury investigation of possible pernicious price fixing between airlines flying across the Atlantic, including BA. President Reagan ordered the investigation stopped last month for unspecified foreign policy reasons, likely related to the strong complaints by the British government of Prime Minister Margaret Thatcher.

The underlying antitrust issue remains unresolved, as the Reagan administration has not given Britain the guarantee it wants against future antitrust prosecutions.

"It's purely a short-term solution for this winter," AP quoted a spokesman for Britain's Department of Transport as saying. CAPTION: Picture, World Airways yesterday received authorization for a new $338 roundtrip fare between Baltimore and London. World Airways Photo