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."
More top news
A bright and breezy day tomorrow with the best of the sunshine through the south and east once early mist and fog patches clear.
Thousands of people have died shortly after being declared fit to work by the Government, new figures reveal.
One billion users logged in to Facebook in a single day for the first time ever, co-founder Mark Zuckerberg announced.