The postal workers' union will ballot to sever links with Labour in the next few weeks unless plans to sell a stake in Royal Mail are scrapped, The Times has learnt.

The move comes as a Bill to sell part of the state-owned group is to be launched in Parliament imminently, much earlier than had been expected.

The Communication Workers Union (CWU) has a conference pledge to ballot on cutting funds to Labour if the organisation moves out of full public ownership. Billy Hayes, the CWU general secretary, said that the union had to hold a ballot before the end of March.

The action comes as political tension over the sale plans, put forward by Lord Mandelson, the Business Secretary, is running high, threatening the biggest rebellion of the Brown Government. A Commons motion opposing the sale has been signed by 139 Labour MPs. Even if only a proportion of those MPs voted against the Bill, the Government would have to rely on the support of the Conservatives for it to become law. Even that support could vanish if the Bill, which could be introduced in the House of Lords first, is modified significantly.

It had been expected that legislation would be prepared by Easter, but the Bill has been largely written and will go to Parliament very soon.

The CWU is one of Labour's largest funders, typically providing about £1 million a year in affiliation fees and local-level funding.

In an interview with The Times, Mr Hayes said: “If our members vote not to give money to the Labour Party, then it would be goodbye. I've been in the party 33 years. It feels a bit like being in an abusive relationship and it comes to that point when you just go.

“There is the old Jack Jones line [about the relationship between the unions and Labour] about murder maybe, divorce never. But as far as I can see there weren't as many divorces in those days. I don't want it to happen, but if we have a ballot I won't be running round the country to say stay in the Labour Party. Why should I?”

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