Proceedings have been issued against party leader Nick Griffin
The UK's equalities watchdog has begun legal action against the British National Party over concerns about ethnic restrictions on its membership.
The Equality and Human Rights Commission said limiting membership to those of an "ethnic origin" described as "indigenous Caucasian" was illegal.
It has issued proceedings against BNP leader Nick Griffin and two officials.
The party called this a "pathetic attempt" by the commission to divert attention from its own problems.
But equality minister Harriet Harman said: "No party should be allowed to have an apartheid constitution in 21st Century Britain. I welcome the action."
The commission has issued county court proceedings against the BNP after voicing concerns in June.
In a statement it said: "The BNP's membership criteria appear to restrict membership to those within what the BNP regards as particular 'ethnic groups' and those whose skin colour is white. This exclusion is contrary to the Race Relations Act.
"The commission believes the BNP's constitution and membership criteria are discriminatory and, further, that the continued publication of them on the BNP website is unlawful.
"It has therefore issued county court proceedings against party leader Nick Griffin and two other officials."
John Wadham, director of legal affairs at the commission, said: "The BNP has said that it is not willing to amend its membership criteria which we believe are discriminatory and unlawful.
"The commission has a statutory duty to use our regulatory powers to enforce compliance with the law, so we have today issued county court proceedings against the BNP.
"However, the party still has an opportunity to resolve this quickly by giving the undertaking on its membership criteria that the commission requires."
The BNP's constitution - last framed in September 2008 - does not explicitly mention the word "white" when talking about restrictions on membership.
The term is only used in the section on the party's political objectives: "It is... committed to stemming and reversing the tide of non-white immigration and to restoring, by legal changes, negotiation and consent, the overwhelmingly white make-up of the British population that existed in Britain prior to 1948."
The BNP's constitution limits membership to a group it describes in this way: "The indigenous British ethnic groups deriving from the class of 'Indigenous Caucasian' consists of members of: i) the Anglo-Saxon folk community; ii) the Celtic Scottish folk community; iii) the Scots-Northern Irish folk community; iv) the Celtic Welsh folk community; v) the Celtic Irish folk community; vi) the Celtic Cornish folk community; vii) the Anglo-Saxon-Celtic folk community; viii) the Celtic-Norse folk community; ix) the Anglo-Saxon-Norse folk community; x) the Anglo-Saxon Indigenous European folk community; xi) members of these ethnic groups which reside either within or outside Europe but ethnically derive from them."
Lee Barnes, legal officer for the BNP, told the BBC: "We think that the commission has brought this action at the behest of the Labour Party.
"It is also a galvanising thing to focus the commission's attention away from its own problems and internal issues."
Four commissioners have have left the EHCR in recent months and there have been criticisms of the leadership of chairman Trevor Phillips.
The communities and local government committee is set to investigate how the watchdog is run later this year and may ask Mr Phillips to give evidence.
Mr Barnes said: "It has nothing to do with discrimination; it's all to do with internal politicking."