Why? Because this:
Three Labour councillors in Birmingham were caught operating a "vote-rigging factory", an Election Court has heard. Police found the trio handling unsealed postal ballots in a deserted warehouse in the city during a late-night raid in June 2004, the hearing was told. Link.
Check out the consprators in this case:
They are Muhammad Afzal, Mohammed Islam and Mohammed Kazi.
On learning that the BNP intended to seal the ballot boxes a Labour spokesman said: "We are confident that when the general election comes the British people will clearly reject the disgusting politics of the BNP. That will be because their politics are vile and divisive not because of any bizarre paranoia about ballot boxes."
Unfortunately the spokesman did not go on to say, "And we promise not to rig any more elections."