A BBC presenter has attacked the decision to invite the leader of the British National Party (BNP) on to BBC1's Question Time tonight, warning that the move "completely contradicts" rules on impartiality and would erode the public's trust in the Corporation.

Michael Rosen, the former children's laureate, accused the broadcaster of "hiding behind" the BNP's European election victories to justify its decision, adding that executives "were relishing" the ratings that the broadcast is expected to receive. "It is failing in the very impartiality that it claims to be trying to uphold," he said.

The BBC Trust is due to be handed a letter from anti-fascist protesters this morning calling for the decision to invite Mr Griffin to take part in the programme to be reversed. However, last night the Trust rejected an appeal by the Welsh Secretary, Peter Hain, to block Mr Griffin's appearance because the BNP had an illegal constitution. The media have been prevented from attending the recording of the show, or even watching a live feed. A spokesman cited "logistical" reasons, but in the past, journalists have been allowed to watch a live feed of the broadcast. The programme will be recorded earlier than normal to allow editors to delete any libellous comments.

Rosen, who presents Radio 4's Word of Mouth, said that the Corporation had "utterly failed in its journalistic duties" in its attitude to handling the BNP after two members who appeared on Radio 1 said the England footballer Ashley Cole was not "ethnically British".

He added that the BBC's championing of the "trust agenda", introduced after controversy over a documentary about the Queen, had been ignored in pushing ahead with Mr Griffin's appearance. "It will lose my trust and that of others if it goes ahead with this," he said. "The BBC is not just any ordinary organisation. It is like a public place – we all own it and need to be a part of it. It has a responsibility to everyone.

"They make this very clear when you work for them. If I were to say anything remotely similar to the things Nick Griffin has said and will say tonight, I would not be allowed on. The BNP's whole notion of reversing immigration rests on the notion that many people shouldn't be here ... the people that the BNP target are licence-fee payers."

He added: "The BBC is obsessed with putting things 'through compliance', to ensure no one will find programmes politically, sexually or socially offensive. I have been stopped from reading a poem that contained one swear word before. Yet while they go into palpitations over things Jonathan Ross says, they are allowing Nick Griffin airtime to say things that will offend millions."

Meanwhile, Mr Griffin welcomed the BBC's decision. "This could be the key moment that propels the BNP into the big time. Never before have we had the chance to present our patriotic, common-sense solutions to Britain's nightmare situation to the public at large in such a prominent fashion."

The BBC's Director General Mark Thompson said ministers would have to impose a broadcasting ban, as it did with Sinn Fein in the 1980s, before it would consider breaching its its "central principle of impartiality." Writing in The Guardian, he said: "Keeping Nick Griffin off air tonight is a job for parliament, not the BBC."

Hundreds of protesters are expected to gather outside the BBC tonight, and police are poised for violent clashes. The programme may be moved to a secret location should it prove too dangerous to ferry guests to the studio in BBC Television Centre, west London.

Weyman Bennett, joint national secretary of Unite Against Fascism, said: "The BBC will be responsible for any increase in racist attacks that take place after they have gone ahead with this."

A spokesman for the BBC said: "The BNP received more than one million votes at the European elections and we do allow parties on once they reach a certain level of support."