Brussels — An early boom in cable television in Europe may be turning to bust. In some countries, a slowing of cable sales stems from a belief that viewers have enough television already. In others, controversies over channel and program content and advertising are causing a drop in the rate of cable hookups.
The development is particularly noticeable in smaller West European countries. In Belgium, the world's ''most cabled'' nation, some 40 cable companies already feed up to 15 channels into the homes of 87 percent of the country's 3 million TV owners. This near-saturation of the market may be one reason why an agreement to pipe BBC television into Belgium by Christmas has run into trouble.
So far, only one of Belgium's three main cable operators has announced it will carry BBC TV under the agreement signed in September. The other two say they will not because of viewer apathy.
Even in some larger European countries the future looks less rosy than it once did. Would-be cable operators in Italy, for instance, point to the country's glut of commercial stations and see no market for new cable programs or advertising.
In France, West Germany, and Britain, however, cable is still booming.
In Scandinavia, political complexities make it certain that no cable boom is in the offing there.
Denmark's conservative government wants to expand the existing cable network with private investment while keeping a ban on advertising. But it may be outvoted by the left in Parliament.
Sweden's socialist government is against lifting a ban on ads. It wants to enlarge the cable system, but plans to stress services other than entertainment, such as electronic shopping.
The problem of transfrontier advertising also is slowing the move to cable in many countries. The French government, for example, is concerned that a proposed deal between the state-owned broadcasting company Telediffusion de France and Luxembourg's Radio-Tele-Luxembourg allowing RTL to be broadcast in France would result in the loss of French advertising revenue to the Luxembourg station.
A recent study, meanwhile, has shown that small-country reluctance or inability to expand their cable systems will mean that in all of Western Europe, cable TV is not likely to reach more than 30 percent of the homes before 1992 - i.e., roughly the level in the United States today.