Prior to 31 December 2004, the wording of section 174 of the Trade Union and Labour Relations (Consolidation) Act 1992 (TULRCA) prevented a union from expelling a union member for conduct which was "being, or ceasing to be, or having been or ceased to be ... a member of a political party". On this wording, an employment tribunal found that ASLEF was in breach when it expelled Mr Lee who was a member of the BNP. The EAT upheld an appeal by ASLEF against this decision and the case was remitted to another tribunal which again found ASLEF was in breach. Having exhausted domestic remedies, ASLEF claimed before the European Court of Human Rights that the UK was in breach of Article 11 (freedom of association) of the Convention for the Protection of Human Rights and Fundamental Freedoms. The ECtHR held that domestic legislation was in violation of the Convention right, in that the balance of competing rights (the union's right to choose its members and the individual's right to join the union) had not been struck.

Following the amendments to section 174 of TULRCA which became effective on 31 December 2004, expulsion for being a member of a political party is still unlawful, but expulsion for conduct which consists of activities undertaken by an individual as a member of a political party is not. Unions may not wish to have members with extreme political views, but they can only expel them for their activities, rather than simply their membership of a political party.