Peter Hain has written to BBC director general Mark Thompson demanding he suspend the “abhorrent” inclusion of the British National Party leader on the flagship political debate show.

The Welsh Secretary argued that the BNP was at present “an unlawful body” after the party told a court last week it would amend its whites-only membership rules to meet discrimination legislation.

The Equality and Human Rights Commission had issued county court proceedings over concerns the membership criteria were restrictive to those within certain ethnic groups.

Mr Griffin is due to appear on Thursday’s edition of Question Time alongside Justice Secretary Jack Straw, representatives of the other main parties and black writer Bonnie Greer.

But in his letter, Mr Hain said: “Now that the BNP have accepted they are at present an unlawful body, it would be perverse of you to maintain that they are just like any other democratically-elected party. On their own admission, at present, they are not.

“If you do not review the decision you may run the very serious risk of legal challenge in addition to the moral objections that I make.

“In my view, your approach is unreasonable, irrational and unlawful.”

Mr Hain, a prominent anti-apartheid activist before becoming an MP, went on: “I believe it is clear that you should now suspend the invitation to see if Nick Griffin is able to agree a new constitution with his party. At that point the commission and the court will be satisfied that the BNP pass a basic threshold of legality.

“In the meantime, surely you have no choice but to rescind the invitation and await the court’s final decision on the matter? You are giving the BNP a legitimacy even they dare not claim in their current unlawful status.”

The BBC says Mr Griffin’s inclusion is based on obligations resulting from the party’s success in winning two seats in European Parliament elections this year.

A spokesman said the corporation had set out its position on the Question Time decision and would respond to Mr Hain “in due course”.

He added: “Our understanding is that, if there was an election tomorrow, the BNP would be able to stand.

“Our audiences, and the electorate, will make up their own minds about the different policies offered by elected politicians.”

Anti-fascist campaigners plan to stage a protest against Mr Griffin’s presence on the show at the BBC’s Television Centre in west London when it is filmed on Thursday.

But an opinion poll at the weekend found voters backed the BBC by 63% to 23%.

Mr Hain, who campaigned against apartheid for many years, said it was wrong for the BBC to treat the BNP in the same way as other political parties.

"The difference between the BNP and other organisations is that the BNP consistently abuses its own freedom of speech to deny it to others by attacking black people, Muslims, or Jewish citizens," he told BBC Radio 5 Live.

"What they do not deserve, in my view - and the BBC has taken an extraordinary decision here - is to put them alongside the other parties as equally legitimate, equally respectable, when we know they are a racist party with fascist roots.

"I think that puts them in an entirely separate box."