The BNP is facing the threat of an injunction from the official body on race discrimination, in what is believed to be the first such action against a political party.

The Equality and Human Rights Commission, the independent watchdog on discrimination, wrote to the BNP today stating that it believes the party is in breach of the Race Relations Act on three counts.

"The legal advice we have received indicates that the British National party's constitution and membership criteria, employment practices and provision of services to constituents and the public may breach discrimination laws which all political parties are legally obliged to uphold," said the commission's legal director, John Wadham.

The letter gives the BNP until 20 July to provide written undertakings in response to the allegations, including a statement that it will not discriminate in party recruitment.

Currently, BNP recruitment is open to members of the party who, according to its constitution, are of … "'indigenous Caucasian' and defined 'ethnic groups' emanating from that Race".

"The commission thinks that this requirement is contrary to the Race Relations Act, which outlaws the refusal or deliberate omission to offer employment on the basis of non-membership of an organisation," a statement released by the commission says. "The commission is therefore concerned that the BNP may have acted, and be acting, illegally."

Other potential breaches of the law raised in the letter include concerns that the BNP's elected representatives may not intend to offer or provide services on an equal basis to all their constituents irrespective of race and their membership criteria.

After the BNP won two seats in the European parliament earlier this month, the Guardian reported numerous grounds for legal challenge against the party. Lawyers said the BNP's rise in public office would have increasing legal significance, including a possible investigation by the commission.

The action by the commission is likely to have serious implications, and could lead to further measures, including an injunction against the party and possible legal challenges in court.

"We await a response from the BNP to our letter before deciding what further action we may take, " Wadham said. "Litigation or enforcement action can be avoided by the BNP giving a satisfactory response to our letter."

The controversial move is the first time the commission has used against a political party new enforcement powers it obtained after taking over from the former race watchdog, the Commission for Racial Equality, in 2007.

The BNP said it had passed the letter on to its legal team.

"We were expecting something like this but we are not too bothered. We are quite happy with our position," a spokesman said.