The RMT rail union will hold a special conference in Glasgow on 6 February to discuss its response to a threat by the Labour Party to disaffiliate it.
The Labour Party's move follows moves by seven RMT branches in Scotland, and the RMT's Scottish Regional Council, to affiliate to the Scottish Socialist Party.
"Last year's [union conference] enshrined our affiliation to Labour in RMT's rules and agreed that branches should be free to support other organisations and campaigns in pursuance of the union's policy objectives, subject to executive approval," says RMT general secretary Bob Crow.
"The Labour Party is now saying that there is a constitutional conflict, and the matter can only be resolved by the union's supreme governing body".
The Labour Party has been willing to ignore the letter of the rules in other cases, for example ignoring unions' support for Ken Livingstone for London mayor even while he was opposing the Labour Party. Here, however, it looks as if the Labour Party wants to drive a wedge between the RMT and other left-leaning unions.
Unfortunately, it is helped by the fact that Bob Crow has already reduced the RMT's level of affiliation to Labour so that it is already little more than nominal.
But activists in the RMT and other unions should demand that the Labour Party withdraws its letter threatening disaffiliation.