In Japan, they know all about the three wise monkeys, who see no evil, hear no evil, and speak no evil – because one has his hands over his eyes, another over his ears, and the third over his mouth.

So if you want to level a gentle insult at a group wilfully ignoring what is in front of them, you can call them the "three wise monkeys" – so long as your audience gets the reference.

But not everyone does, as four activists in public sector union Unison have discovered. They have been disciplined for distributing an "offensive" leaflet which carried a drawing of the proverbial monkeys. The cartoon was interpreted as a racist slur against a fellow activist – a charge the four vehemently deny. They say no one could believe they intended a racist insult, and that they are victims of a "witch hunt" by New Labour loyalists, who control the union. The four have been barred from holding office in the union, but have taken their case to appeal – which begins today.

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The row dates from an incident nearly two years ago, when supporters of the Socialist Party – a Marxist group well to the left of Labour – tried to get Unison's annual conference to discuss proposals to cut links with the Labour Party, and reduce the pay of full-time union officials. Unison is one of the biggest donors to Labour, despite occasional friction.

The rebels were told by Unison's standing orders committee, which determines what can be discussed at conference, that there were more important matters to be debated in the available time. Irritated by this, they distributed a hastily printed leaflet with a cartoon that compared the committee with the three wise monkeys.

The only member of the committee familiar to most Unison members is Clytus Williams, the chairman. He is a lay union activist and well liked by most delegates. He is also black. The idea the far left was trying to caricature him as a monkey infuriated delegates, many of whom are low-paid workers more familiar with bar room racism than Buddhist culture and proverbial monkeys.

Four members of the Socialist Party, Glenn Kelly, Onay Kasab, Suzanne Muna and Brian Debus – who all hold unpaid offices in Unison branches – were ruled to have distributed "offensive" material, and barred from office – though investigators accepted there was no "racist intent". Their cause has been taken up by comedians Rory Bremner and Mark Thomas. Mr Bremner said: "On the face of it, Unison are about to make themselves a laughing stock. They need to be aware of the ridicule this will attract to the union and its leaders, and think again before the papers get hold of this and make them look foolish and authoritarian."

Mr Thomas said: "I know Onay Kasab. To accuse him of being racist is utter stupidity and madness. Those who brought this charge need to take a lie down in a quiet room, possibly with whale music playing and get a rest."

Labour MP, John McDonnell, a veteran critic of New Labour, said: "Unison's leadership are doing the bosses a favour. It is a political witch hunt."

A spokeswoman for Unison said: "We confirm a National Executive Council panel found these members breached several Unison rules, which require 'members, activists, representatives and staff, are treated with dignity and respect at all times when participating in the union's democratic structures'. As there may be an appeal, the union is unable to add more until this is concluded."