High profile MP Tom Watson has vowed to 鈥渇ight very hard鈥 to ensure Labour keeps its historic links to the trade unions.
The Black Country MP spoke out as Labour leader Ed Miliband faced a battle with the unions over party funding.
The GMB union announced it was cutting its funding for Labour from 拢1.2 million a year to just 拢150,000.
It follows Mr Miliband鈥檚 announcement that he wants to change the way unions support Labour.
Funding from union members who choose to pay an additional fee known as the political levy, which can be used on political campaigns, can currently be donated automatically by trade unions to the Labour Party.
Mr Miliband wants to change this so that union members must 鈥渙pt in鈥 to backing Labour by becoming associate members of the party before they can contribute funding.
But unions, and some Labour MPs, see this as a way of weakening the historic link between Labour and the unions.
It follows the row over the selection of a Labour candidate in Falkirk, Scotland, where the UNITE union 鈥 which denies doing anything wrong 鈥 was accused of signing up members so it could have undue influence over the decision.
The successful candidate was Karie Murphy, Mr Watson鈥檚 former office assistant, but she has been suspended from the party while Labour officials investigate the selection.
Mr Watson resigned from his role as Labour鈥檚 campaign co-ordinator following the row, although he played no role in the selection.
The MP has now published an article on his website demanding that Labour retain its links with the unions.
He said: 鈥淚f this is the beginning of the end of that historic link, it is a very serious development that threatens a pillar of our democracy that has endured for over one hundred years.
鈥淪ome will scoff but they are fools to do so. That party card stands for something more than confirmation that an annual direct debit has been processed.
鈥淥ver the next year we have been asked to consider a change to the constitution of the Labour party, though no detailed proposals have been revealed. I鈥檓 not opposed to reform but I will fight very hard to retain the fundamental link between the party and Labour movement.鈥
Labour is planning to hold a special conference next spring to finalise details of the changes.
Labour MP Ian Lavery has described the affiliation reform as the 鈥渂iggest political gamble鈥 in the history of the party.
Mr Lavery, who chairs the trade union group of Labour MPs, said he believed fewer than 15 per cent of union members would opt to join Labour under the changes.
鈥淧eople are not queuing up to join Labour 鈥 quite the opposite. They are waiting to see what the party will bring to the table in its manifesto,鈥 he said.
Shadow Treasury secretary Rachel Reeves said she was 鈥渃onfident鈥 more trade union members would sign up under the new system.
鈥淢ost of the money that the Labour Party receives comes from small donations and members,鈥 she said.
鈥淥f course we welcome the support that we get from the trade unions but this is a decision for the GMB.
鈥淓d has spoken about the need for greater transparency and openness in that relationship and we are going to be doing more to reach out and get affiliations from individual members directly to the party.鈥
She went on: 鈥淪ince the last general election, membership in my constituency party in Leeds West has increased by 50 per cent. We鈥檝e got more members, more trade union activists getting involved in the party because they like what Ed Miliband has to say.
鈥淪o I am confident that more people will sign up, get involved in the Labour Party, and come out campaigning. But that openness and transparency is really important and that is what Ed is trying to do.鈥
