Protestors determined to see the closure of London's showpiece City Airport are marshaling forces this week for a public inquiry that will decide whether the tiny Docklands airport has reached the end of the runway.

The future of London City Airport, opened in October 1987 as Europe's first international inner-city airport, now hangs in the balance as pro and anti factions gather support for the hearing due to start on July 3. The outcome may not be known for months.London City has lost its owners John Mowlem and Co. some 9 million ($14.8 million) on operations since it opened in a blaze of publicity as the doorstep airport for London's burgeoning Docklands business community.

London City Airways, one of the two original carriers using the airport, pulled out of one of its routes in March and laid off 30 of its 150 staff in a bid to stem deepening losses.

While passengers and freight users find the airport's small size a distinct advantage - air courier shipments, for instance, can be on board a plane within five minutes of receipt - lack of routes and poor road and rail access have dogged the terminal.

The debate concerning London City's future hinges on whether British Aerospace BAe 146 jets will be permitted to land there. All current flights are by 50-seater Dash-7 aircraft with a useful range limited to 400 miles. With bigger, 1,000-mile 146 jets, the capitals of the Mediterranean and Eastern Europe would come within the airport's operating orbit.

Mowlem and Co. now says it almost certainly will be forced to shut down London City if opponents succeed in banning the 146 from landing there. With a go-ahead, they say profitability is assured.

Despite the question mark hanging over its future, some airport users remain optimistic that government inspectors chairing the inquiry will give them the backing they need.

"We are confident the outcome of the inquiry will be in our favor. We'll then need six to eight months to extend the runway and could see the 146 landing here almost at once," said John Horne, assistant airport director. ''It would give us much more flexibility in terms of both cargo and passenger handling."

Also optimistic is Dash-7 operator Brymon Airways, which enjoyed a 34 percent rise in passenger traffic in February and said its flagship Paris route has been profitable for the past 18 months.

"It's a hard slog, but we appear to be winning. We're now averaging 7,000 passengers a month and are on target to carry 75,000 this year as a whole," a spokesman said. "We expect to cover all our initial investments by next spring."

Air freight at London City is handled chiefly by Elan International, a subsidiary of air courier giant DHL. It currently flies an average of 200 dutiable consignments a day into London City as well as several thousand documents.

Most incoming shipments can be cleared within the hour, a boon for London TV and media companies that import newsreel cassettes and newspapers daily

from Paris and Brussels.

"One factor on our side is that London City is very much a businessman's airport - and businessmen travel light," Mr. Horne said. "That enables us to carry up to 500 kilograms (1,100 pounds) of air freight per flight, which is not bad for a small plane. If we win a go-ahead for the 146, then we'll obviously be able to handle significantly more."

One disadvantage of the airport for freight users is that dutiable consignments valued at over 600 must be cleared at the Port of Tilbury, some miles away.

Clearance could be speeded up, however, if the electronic ACP 90 system used at much larger Heathrow Airport comes to London City. But like many other matters, its installation hinges largely on the persuasive powers of Mowlem and the airport's other protagonists July 3.