The link between Britain's biggest rail union and the Labour Party has been broken after more than 100 years.
Bob Crow: "We don't accept threats"
It follows the Rail, Maritime and Transport union's decision on Friday to continue to allow branches to affiliate to other political parties.
The union rejected an ultimatum from Labour to stop supporting the Scottish Socialist Party (SSP).
The RMT was disaffiliated from Labour at noon on Saturday. Their link dates back to 1899.
In its original guise as the Amalgamated Society of Railway Servants, the RMT helped found what is now the Labour Party.
The union's general secretary Bob Crow said that he was very sad about the move and that the RMT never wanted to be expelled by the Labour Party.
He added: "As far as we're concerned our members (in Scotland) took a
"I don't think it's right for anyone to tell us what to do."
Mr Crow said that the RMT never had an opportunity to put its case in a
disciplinary hearing before it effectively left the Labour Party at midday on Saturday.
He said: "We got a noon deadline. We don't accept threats from an employer and we're not going to accept one from the Labour Party.
"If a branch wants to take the decision to affiliate with the Scottish
Socialist Party, that's their right."
Asked if other unions would follow in the RMT's wake, Mr Crow replied: "What
other unions do is up to them.
"We wouldn't get involved in their affairs and we wouldn't expect them to interfere with ours."
Labour Party chairman Ian McCartney told BBC News Mr Crow had long ago decided to cut ties to Labour.
He said: "It's a complete nonsense. He knew what he was doing but he didn't have the guts to tell his members."
Mr McCartney said other union leaders were begging Mr Crow not to continue the feud.
He added: "Tens of thousands of his members will be angry about the way in which this has happened."
The link between Labour and the RMT was formed in 1899
However, SSP leader Tommy Sheridan said the decision "ushered in a new era of development in socialist politics".
He said: "For over 100 years trade unionists have supported socialist policies but New Labour's big business agenda is the enemy of socialist politics and more and more trade unionists are willing to admit this reality.
"The Scottish Socialist Party is a young party and small in comparison to Labour but we must not forget that Labour started life as a small party as well."
He appealed for other trade unionists to switch to the SSP.
The dispute began last year when the RMT, unhappy with some of Labour's policies, allowed five of its branches in Scotland to affiliate to the SSP.
Two more branches and the Scottish Regional Council now also want to switch affiliation.
At a conference on Friday, RMT delegates voted by 42-8 to uphold the decision.
The Fire Brigades Union is also expected to come under pressure to leave Labour, while a branch of the Communication Workers' Union representing 4,500 Scots is also expected to affiliate to the SSP.