Households across Japan are threatening to cancel payment of the television licence fee in protest at an insider dealing scandal at NHK, the austere state broadcaster.

The nationwide campaign, expected to begin next week, could cost NHK £300 million in lost revenues.

The planned protest comes after government demands for financial regulators to broaden the investigation of NHK journalists. Hiroya Masuda, the Communications Minister, said yesterday that all 5,000 employees of NHK's news section should be added to the inquiry.

Three senior business journalists at NHK are under suspicion for allegedly buying shares in a chain of restaurants 20 minutes before the broadcast of a highly positive, exclusive news story about a capital tie-up between two restaurants. The reporters are said to have seen on an internal editing system an interview recorded earlier in the day with the president of Zensho, the buying company. They are alleged to have left the building and used mobile phones to make substantial investments in the target company, Kappa Create. When the takeover scoop hit the airwaves the shares soared by 13 per cent. Two of the three reporters have admitted the insider trades; the third denies the charge.

Mr Masuda said he suspected that there may be more NHK journalists involved in the scam than the three identified so far and that the corporation's internal investigation was likely to be ineffective. “I've been told that all NHK employees are doing all they can to be compliant, but I doubt it,” he said.

The Japanese public see the insider dealing scandal as the “final straw” after a string of bizarre lapses by NHK. Last week viewers were terrified when a computer glitch resulted in them being bombarded with information about an earthquake that had not happened. Moreover, many have not forgiven NHK for its new year variety show that heralded the start of 2007 in appalling fashion - designed for the family audience, it showed an act involving 100 dancers in suits that made them appear naked.

The latest revelations have shredded NHK's image as the responsible voice of Japanese news reporting and senior officials at the corporation say that they are braced for widespread refusals to pay this year's first licence fee demand.

Agonised apologies and vows of harsh reprisals from Genichi Hashimoto, the president of NHK, have done little to calm public fury. Yesterday the company's switchboard was jammed with complaints from viewers, who described the journalists' actions as “unforgivable” and said that they would refuse to pay the monthly licence fee.

NHK's financing depends on a delicate combination of the fee and an annual funding injection from the government. Non-payment of the licence fee is not a crime: a trust system for payment has traditionally prevailed and for many years avoidance could be achieved by simply not opening the door to collectors.

Refusal to pay now involves little more than cancelling a monthly direct debit, which means that it is easy for viewers to register their protest. In 2004, outrage at an NHK embezzlement scandal provoked a fee strike that cost more than £200million in lost revenues and the public anger this time is viewed as far worse.

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