Labour faces losing a big union backer as the rebellion against Lord Mandelson’s plans to sell part of the Royal Mail gathers force.

The Communication Workers Union (CWU) will ask its 250,000 members to approve a formal split from the party if the Business Secretary presses ahead, The Times has learnt. A ministerial aide resigned in protest yesterday, delivering a warning to Gordon Brown that he faces a serious revolt over the issue.

Jim McGovern quit as Parliamentary Private Secretary to Pat McFadden, the Post Office Minister, after the publication on Tuesday of a report recommending an injection of private capital. The MP for Dundee West said that he could not support “what looks to me like partial privatisation of the Royal Mail. Pat McFadden said he welcomed an expression of interest from the Dutch postal company TNT. For me, it simply beggars belief that we would employ the services of a company from abroad to tell the Royal Mail where it is going wrong.”

Lord Mandelson insists that allowing a minority holding does not breach Labour’s manifesto obligation not to privatise the postal service. He will seek to reassure backbenchers at a meeting today.

Times Archive, 1929: The Post Office from within

The Post Office cannot prosper until it is set free from a form of control which is fatally inappropriate to a commercial organisation

• Post Office and Public

• Letter from the Postmaster General

• The Post Office: Grounds of public discontent

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Alan Johnson, the Health Secretary and a former general secretary of the CWU, dismissed suggestions that he opposed a partnership between Royal Mail and TNT. He is understood to have spoken in favour of the plans in Cabinet on Tuesday, insisting that they represented the postal service’s only hope of survival.

Declining volumes of mail and a rocketing pensions deficit have made some sort of private partnership vital, ministers believe. They promise full consultation over the sell-off — postponing a Commons clash until well into next year.

Lord Mandelson faces a more immediate threat from the CWU, however, since the union is committed to a ballot on splitting from Labour in the event of any attempt at privatisation. Its annual conference this year passed a resolution saying that the union “reaffirms its opposition to privatisation in any form. This includes outright privatisation, joint ventures, splitting the business and allowing private companies to take over parts of it or infusions of private capital”.

A Labour MP close the union said yesterday that it was likely to call a special conference and hold the ballot unless Lord Mandelson backed down.

The loss of the CWU — which has given Labour more than £5 million since 2001 and £500,000 over the past 12 months — would be a blow and might trigger supportive action from other unions.

A move to disaffiliate would mark the third split between Labour and the unions since 1997. But opposition from the CWU is potentially far more damaging than the previous splits by smaller unions because the CWU represents workers across a number of large industries. In 2004 the Rail, Maritime and Transport (RMT) union was expelled by Labour for backing the Scottish Socialist Party. Later that year the Fire Brigades Union (FBU) voted for disaffiliation after the long and bitter firefighters’ strike of 2003. Although the RMT and the FBU are industrially powerful, their membership is a fraction of the CWU’s.

Royal Mail workers are already taking action from tomorrow over plans to close sorting centres. Workers in Crewe, Coventry, Bolton, Stockport and Oxford will stage a 24-hour walkout. A strike in Liverpool was called off after an injunction was served.

Brendan Barber, the TUC General-Secretary, said: “The Government says that any joint venture doesn’t represent a watering-down of that commitment, but these proposals will raise fears that they are a step on the way to full privatisation. While moves to underwrite the pension deficit and a commitment to long-overdue modernisation are very welcome, privatisation cannot be an option.”

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