Britain appeared headed toward a nationwide broadcast news blackout Wednesday as journalists from the country's Independent Television News voted today to join a 24-hour strike by colleagues at the British Broadcasting Corp. to protest cancellation of a controversial BBC documentary on Northern Ireland.

In addition to ITN, the strike action now includes all BBC television and radio journalists broadcasting in England and Scotland, as well as BBC World Service radio news broadcasts overseas. Although many journalists already had voted support in their individual union chapters, the National Union of Journalists sent provisional instructions to all BBC members today to honor the strike.

That order is likely to be approved at a meeting of the union's emergency committee on Monday. Other local independent television stations, as well as technicians' unions, also are due to vote on the action. Those now likely to observe the strike include workers on all afternoon and evening national news programs on all four television channels in Britain as well as BBC radio.

The one-day strike was organized by BBC television journalists in London this week after the corporation's board of governors agreed to an unprecedented public government request to cancel a documentary on Northern Ireland. Home Secretary Leon Brittan charged that the program, which features an interview with a man believed to be chief of staff of the outlawed Irish Republican Army, would promote terrorism and was "against the national interest."

The journalists, supported by civil liberties and press freedom organizations as well as opposition political parties here, have accused the government of censorship and charged that the board's decision has damaged their credibility and reputation for editorial independence. The controversy also has provoked widespread public dialogue here, with newspapers running numerous letters from readers. While most appear to support the journalists, a number have condemned them for publicizing terrorists and called for all reporting on the IRA to be banned.

The program long had been approved by the BBC management under its own internal guidelines for programs dealing with the sensitive Northern Ireland issue.

Brittan denied he had tried to censor the BBC, and said that if the board had decided to go ahead with the documentary despite his "request," he would not have invoked his legal powers to ban it.

The strike is due to begin at midnight Aug. 6, lasting throughout Aug. 7, the date the program was scheduled for broadcast.