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GMB general secretary Paul Kenny said he "can't understand what all the fuss is about" after the union decided to cut funds to the Labour Party.
GMB union general secretary Paul Kenny and Ed Miliband are due to meet tomorrow in a meeting planned before today's decision to cut funding to Labour by £1 million.
A union source said its was made out of sorrow rather than in anger.
However, according to ITV News political correspondent Carl Dinnen, there are those in the Labour Party who thought it was a deliberate snub ahead of tomorrow's meeting:
Total funding for the Labour Party in the year to June 2013 was £16.8 million, which £4.2 million came from trade unions, figures released by the party show.
Former home secretary Alan Johnson - an ex-union leader - warned against "petty retribution" in the wake of Ed Miliband's party reform pledge.
"We can no longer go on living the lie with these millions of people signed up as levy-paying members to the Labour Party, and treated as if they were members," Mr Johnson told BBC Radio 4's The World At One.
"I call them the ghosts in the machine. That is bad for the party, it is bad for the trade unions."
The former Communication Workers Union chief added, "I just hope this is not a piece of petty retribution by a trade union."
A senior Labour source said the party had been given "more than a few hours" notice of the GMB's decision to reduce its funding, and was not aware of any other unions planning similar moves.
Labour leader Ed Miliband will not be deterred from his plans to "mend not end" Labour's financial links with the unions, the unnamed source added.
A Conservative Party spokesman said: "Nothing has changed. The union barons can still rig Labour's candidate selections, buy Labour's policies and pick Labour's leader.
"If Ed Miliband is too weak to stand up to his union bosses, he will be too weak to stand up for hardworking people."
Labour's former General Election co-ordinator Tom Watson has pledged to "fight very hard" to retain the party's links with trade unions after the GMB union announced it was slashing its funding.
Mr Watson, who was involved in the selection of Labour's election candidates, resigned his shadow cabinet position over the Falkirk row.
He wrote on his blog: "If 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.
"Some 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."
Shadow Treasury secretary Rachel Reeves said she was "confident" more trade union members would sign up to the party when automatic affiliation is stopped.
"Of course we welcome the support that we get from the trade unions but this is a decision for the GMB," she told BBC Radio 4's Today.
"Since the last general election, membership in my constituency party in Leeds West has increased by 50 per cent. We've got more members, more trade union activists getting involved in the party because they like what Ed Miliband has to say.
"So 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."
The GMB union is to reduce its spending on Labour Party campaigns and initiatives. It comes as the union announced it was to cut its affiliation funds in the wake of the row over party reforms.
The decision follows plans by Ed Miliband to give individual union members the choice of opting to join the party rather than being automatically affiliated. The union has affiliated 420,000 of its members to Labour but that figure will be cut to 50,000 from January.
The GMB union is to cut its affiliation funds to Labour from £1.2 million to £150,000 in the wake of the row over party reforms, the union has announced.
Latest ITV News reports
A union that supported Ed Miliband's election as Labour leader has decided to cut its funds to the party from £1.2 million to £150,000.