LONDON -- Two years after it was introduced, Britain's prohibition against the broadcast of statements by members of purported terrorist organizations and their sympathizers remains a source of controversy and occasional embarrassment for a country considered one of the world's foremost democracies.

The latest incident occurred last month when a television company removed the voices of two of Ireland's most revered statesmen -- the late prime minister Eamon de Valera and late Nobel Peace Prize winner Sean MacBride -- from a history program to be broadcast to schoolchildren in Northern Ireland. MacBride's voice was banned because he was once a leader of the outlawed Irish Republican Army, De Valera's because he once was president of the IRA's political arm, Sinn Fein, a legal but "listed" political organization.

Similarly, a pop song calling for the release of the Birmingham Six, whose terrorist bombing convictions are under legal challenge, was banned from radio use by the Independent Broadcasting Authority, which ruled that the song might attract sympathy to the IRA's cause and undermine governmental authority.

The comments of other politicians such as Sinn Fein leader Gerry Adams, who is an elected member of Parliament, are routinely dubbed, presented with subtitles or go unreported altogether because of the ban, which critics compare to broadcast restrictions in countries such as South Africa and Israel.

"It's a complete farce," Scarlett MccGwire, a television documentary producer, said at a rally marking the ban's second anniversary Friday. "We must be the only country in the world where we have a totally legal political party with a member of Parliament and local councilors, and we're not allowed to hear them speak." MccGwire is among a group of journalists who have filed suit challenging the ban.

Even its most vocal critics concede the ban has been effective. The Glasgow University Media Group reported last week that televised appearances by leaders of Sinn Fein fell from 93 in the year before the ban to just 34 in the 12 months after it was imposed.

"There will be gray areas and occasional isolated incidents where the prohibition might appear to be skirted, but generally we think it's been very effective and there are no present plans to change it," said Jonathan Haslam, a spokesman for the Home Office.

Sinn Fein spokesmen generally agree that the ban has worked. "It has not succeeded in the goal of marginalizing Sinn Fein's support in Northern Ireland," said Richard McAuley, the party's director of publicity. "But it has worked in preventing people in Britain from getting a true picture of what's going on in the North."

McAuley contends that journalists are partly to blame for not aggressively challenging the ban. "They could run a horse and cart through this ban, but they don't want to become embroiled in controversy with their editors and producers, so they tend to go for the easy option," he said. "That means our point of view doesn't get on television."

The British Broadcasting Corp. and the rival Independent Broadcast Authority both have publicly opposed the ban, but have not joined the lawsuit against it because they say their lawyers have advised them the case would fail in British courts. Instead, the two bodies have regularly lobbied the Home Office against the ban and spoken publicly against it.

John Birt, deputy director general of the BBC, issued a statement Friday contending that the ban "prevents broadcasters capturing the full reality and texture of events and issues in Northern Ireland {and} undermines the BBC's independence to formulate and carry out its own editorial policy."

But critics contend the two bodies have in effect acted as censors for the government by enforcing the ban. "One of the most astonishing things is the gutlessness of the editors," said Tony Benn, a left-wing Labor Party member of Parliament. "Repression has two halves: the will of those who oppress and the willingness of those who are oppressed."

The ban was imposed in 1988 by Douglas Hurd, then the home secretary, after a year of rising confrontation between the government of Prime Minister Margaret Thatcher and the media over coverage of Northern Ireland and the IRA, which has waged a long and violent campaign to end British rule there.

Under the ban, television and radio programs cannot broadcast direct statements from leaders or spokesmen for 11 organizations -- some nationalist, others unionist -- that have either been outlawed by the government as terrorist groups or, like Sinn Fein, deemed sympathizers.

The idea, Thatcher said at the time, was to deny terrorists "the oxygen of publicity." Hurd denied the ban amounted to censorship of journalists. "This is not a restriction on reporting," he told Parliament. "It is a restriction on direct appearances by those who use or support violence."

Critics contend that there was no compelling need for the ban -- television coverage of the IRA and Sinn Fein was generally negative and tended to decrease sympathy for their cause rather than increase it, according to the Glasgow University study.

Some civil libertarians contend the ban is part of a growing pattern of government efforts to restrict press freedom. Francis D'Souza, director of Article 19, an anti-censorship group here, cites a number of cases, including the recent tightening of the Official Secrets Act to eliminate the so-called "public interest defense" for civil servants accused of leaking classified information, the $9,000 fine meted out earlier this year by a London court to a British reporter for refusing to comply with a judicial demand to divulge his sources, and a new broadcasting bill that critics contend will increase official control over newscasts.

"We live in a culture of secrecy," said D'Souza. "The most serious human rights issue in the United Kingdom today is the question of freedom of expression."