Last month the BNP scrapped its whites-only policy after the threat of an injunction from the Equality and Human Rights Commission (EHRC).
But the party still required new members to sign up to its principles.
A judge at Central London County Court has ruled the party’s new constitution is still discriminatory.
Judge Paul Collins ruled: “I hold that the BNP are likely to commit unlawful acts of discrimination within section 1b Race Relations Act 1976 in the terms on which they are prepared to admit persons to membership under the 12th addition of their constitution.”
BNP leader Nick Griffin was met with protests from anti-fascist groups as he arrived at court earlier.
Speaking ahead of the hearing, Mr Griffin said: “The constitution was always legal.”
During a day of legal submissions on Tuesday, the BNP was accused of “indirectly” discriminating against black and Asian people even though the party no longer bars them from joining.
The BNP denied the allegations and said it had a “waiting list” of black and Asian people and would welcome more applications from ethnic minorities.