One of the unions that helped found Labour has been warned it may have to leave the party because of a row over the rules.
Bob Crow has demanded the right to appeal the decision
Rail union the RMT has threatened legal action, after the Labour party decided it had breached the rules by allowing affiliation to other parties.
Party chairman Ian McCartney urged the RMT to reverse its policy, but said it was not being "kicked out".
RMT leader Bob Crow said it would look at all options to reverse the ruling.
The union will hold a special conference on 6 February to decide its next move, but Mr Crow warned legal action was one of the options being considered.
The union has been told it will have placed itself outside the party if it doesn't reverse its policy.
Labour has asked the RMT to confirm its intentions by noon on 7 February, the date from which disaffiliation from the party will apply.
Mr McCartney met Mr Crow on Monday following the decision by the union's executive to allow branches to affiliate to the Scottish Socialist Party (SSP).
Mr McCartney said the RMT had clearly breached party rules and he wanted to make
sure that everyone attending next week's special conference was aware of the
"There has been a huge effort to try to persuade the RMT not to kick themselves out of the party. We don't want the RMT to disaffiliate.
"They are valued members and we want them to remain affiliated but only they can make that
He said unions could not support organisations which stood against the Labour
Party in elections.
Five branches of the RMT, the UK's largest rail union, have already been given permission to join the SSP.
On Tuesday members of Labour's National Executive Committee heard a report on the union's decision.
An overwhelming majority voted that the RMT had breached key rules, breaking its constitution.
Only two members of the 33-strong executive voted against, including Mick Cash, the RMT'S deputy general secretary.
The union has been reducing its financial support to Labour in recent years and now only gives the party £12,500 a year.
Their historic link was formed in 1899, when a forerunner of the RMT voted at a conference to set up the Labour Party.