The BNP has voted to admit non-white members but still requires them to sign up to its principles, the Central London County Court was told.
A judge will rule on Friday whether the new rules contravene race laws.
The BNP, which voted on the new rules last month, denies its planned new constitution is discriminatory.
Party members backed changes to its constitution to allow "non-indigenous Britons" to join, after the Equality and Human Rights Commission (EHRC) took legal action.
'Disadvantage'
The court heard that prospective members had to sign up to principles including a duty to oppose the promotion of any form of "integration or assimilation" that impacted on the "indigenous British", and a requirement to support the "maintenance and existence of the unity and integrity of the indigenous British".
Robin Allen, QC, representing the EHRC, said: "That is something which we would submit is indirectly discriminatory.
"They will put persons who do not fall into the indigenous British category at a disadvantage."
He argued the principles could be interpreted to oppose mixed marriages and could force people to deny their own identity.
Mr Allen said: "We simply say from the commission's point of view we are statutorily obliged to encourage diversity and recognise diversity as part of the British state in its widest sense.
"We're indifferent as to the expression of views as long as they are lawful. It's the condition of access that we take exception to."
'Welcome'
The new, 12th, version of the BNP constitution states that members have to agree to two party officials - one male and one female - visiting their home for up to two hours, the court heard.
Mr Allen said this could be used to enable potential members to be intimidated, although there was no evidence it had been used in that way.
But Gwynn Price Rowlands, for the BNP, told the court there were "significant numbers" of members who were mixed race or in mixed marriages, along with "several" Jewish members.
He said: "They (the BNP) make it clear that they would welcome more applications from ethnic minorities."
Mr Rowlands told the court he "could not understand" how a black or Asian person supporting the concept of British nationalism could be discriminatory.
He added that the BNP was "simply putting their principles forward and allowing any ethnic minority member to join and support and espouse those principles".
Judge Paul Collins said he would issue his judgement on the issue on Friday.
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