LONDON — HAVING turned television into high art through such programs as Masterpiece Theater, Britain has also made progress in scrubbing its airwaves of scenes depicting violence. But not enough to satisfy critics and government officials seeking new ways to stem TV mayhem. And it could be that the Ninja Turtles and Power Rangers have met their match in the British Cabinet minister who oversees broadcasting, Virginia Bottomley. She is likely by early next year to insist on ''taste and decency'' standards for the British Broadcasting Corporation (BBC) and British commercial channels, according to her officials. A recent survey found programs showing ''an intention to harm or to intimidate'' characters portrayed on TV - many of the shows imported from America - have been cut nearly in half over nearly the past decade. Researchers at Sheffield University near Manchester found that violence on Britain's four terrestrial TV channels had fallen from 1.1 percent of all content nine years ago to 0.62 percent today, based on analysis of 4,715 hours of programs over six months. Despite the progress, points out Cabinet minister Bottomley, 37 percent of programs still contain some violence, with 19 percent of it in children's shows. In the programs containing violence, the report says, there were 21,000 separate violent acts depicted in 10,000 sequences. Violent scenes were twice as common on satellite channels than on terrestrial channels. Satellite TV is steadily becoming more popular in Britain as media magnate Rupert Murdoch succeeds in persuading more and more homes to install receiving dishes for his British Sky Broadcasting (BSkyB) programs. Violence on British TV is hotly debated by broadcasters and citizens' groups. They frequently find themselves at odds over how much violence there is on television and what effect it has on viewers - particularly the very young. Two years ago the issue received huge attention when it was revealed that two children who had beaten to death a toddler and left his body on a railway track had been watching a video showing similar scenes only a short time earlier. While Bottomley is expected to go on the offensive against violence on the BBC and Britain's terrestrial channels, it is less clear how she is going to persuade BSkyB to cut back on violence in its transmissions. The company is headquartered outside Britain and is not subject to British broadcasting legislation. ''The government takes very seriously public concern about the portrayal of violence on our TV screens,'' Bottomley said last month. ''Broadcasting is a powerful medium with potential to do great good, but we must be vigilant against its negative influence.'' The Sheffield research - the most detailed survey in recent years - was commissioned jointly by the BBC and the Independent Television Commission, which regulates terrestrial commercial TV channels. It found that about half of all violence on TV occurred in feature films, mostly American in origin. The amount of violence in TV imports was twice as high as in home-produced programs. Barrie Gunter and Jackie Harrison, the report's authors, say it is important to be clear that differing types of violence on screen have different effects on an audience. Mr. Gunter says that for content analysis to be meaningful, ''it must be accompanied by research on how viewers react to different kinds of violence.'' Against the background of public concern, the government has persuaded the main broadcasting organizations, including BSkyB, to produce an industrywide 28-day monitoring study next year. A separate three-year survey of TV's impact on children is already under way. Pressure to clean up British TV is coming from several citizens' groups and from the government-sponsored Broadcasting Standards Council. The most influential citizen's organization is the National Viewers' and Listeners' Association, set up in the 1960s by the moral crusader Mary Whitehouse. Its chairman, Graham Stevens, says that the latest research reveals only ''modest progress.'' ''Whatever the statistics, there is too much violence on television, and it needs to be curtailed,'' he says. The Sheffield study has revealed some thought-provoking facts. Most violent acts depicted on TV were in present-day settings and inner-city locations. The majority of perpetrators and victims were young white males. Even more striking is the breakdown of motives for violent acts: * Pursuit of crime: 25 percent. * Personal relationships: 23 percent. * Law enforcement: 8 percent. * Defending civil liberties: 7 percent. * Acts of war involving military forces: 6 percent. * Domestic: 4 percent. * Other: 27 percent. Philip Graham, chairman of the National Children's Bureau, a watchdog body, strikes a note of caution about the finding that ''only'' 0.6 percent of all programs portray violence. ''These are likely to be the programs young people will watch,'' he says. ''It is like football [soccer]: Goal-scoring may account for only a fraction of a match, but it is the goals that people remember.'' As well as trying to curb violence on British screens, Bottomley is trying to convince broadcasters that the ability of the medium to shock can be socially beneficial. She says her mailbag contains many letters from citizens demanding action on issues raised by television. A recent documentary about orphanages in China where sometimes baby girls are left to die produced many letters to the government calling on it to take the issue up with the Beijing authorities, she says. Equally, however, Cabinet minister Bottomley is determined that TV should exercise restraint and self-control, commenting: ''Questions of good taste and decency can never be taken for granted.''