Tomorrow the Employment Bill reaches its final stages in the House of Commons. Section 18 of the original Bill would have allowed trade unions to exclude members of the British National Party and other racists. The Bill that will be presented to MPs tomorrow, however, has a very different version - one that would achieve the precise opposite of the Bill's initial intent. The House of Lords altered it so greatly that it now protects BNP members' union membership. Tomorrow Labour MPs will propose an amendment to restore the original section allowing unions to expel BNP members.

This is not, as it has been presented, a question of trade union rights. Rather, the issue at stake is the very notion of freedom of association. It is in everyone's interest - even, counterintuitively, that of BNP members - that the amendment succeeds, and unions are permitted to decide who may or may not join them.

In law, trade unions ought to have no greater rights than any other bodies. But nor should they, in their internal affairs, have any greater responsibilities. The law should step in only when a private body acts in a way that affects the rest of us.

When unions chose not to ballot members on strike action, the law rightly stepped in. But it ought to be no concern of the State how a union structures its private affairs. If it decides to allow membership only to workers who live in Lancashire, that should be its business alone; it is not the State's role to tell it to offer membership to workers in Yorkshire, too. Similarly, the State should not tell unions that they must allow BNP members to join.

But if it is right that unions should, as private members' organisations, be able to decide for themselves if they wish to include members of the BNP, the same should apply to all other private bodies. If a group of women want to set up a women-only lunch society, the law should not dictate that membership must also be open to men, and vice versa - so long as those groups are private bodies whose activities have no significant impact on the rest of us.

As it stands, the Employment Bill's protection of BNP members' right to union membership is another extension of state interference in freedom of association. Were the BNP a more serious organisation, it would see that this could threaten its own existence. Repellent as the BNP may be, racists too have a right to gather together. Their behaviour should only concern the law when it impacts on others.

All MPs who care about freedom of association - whether it extends to unions, clubs or racist parties - ought to support the proposed amendment to the Bill tomorrow.

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