"The Royal Mail is a much-cherished national institution," he told lawmakers in the House of Commons.
"The case for privatization has not been met. With recent annual profits of over 400 million pounds, we should be allowing it to flourish in the public sector."
Even as the announcement of the IPO was made, the Communications Workers Union was planning to ballot workers for possible strike action. The union is opposed to the stock plan because of fears of potential job losses.
There are concerns the threat of industrial action may damp demand for the stock.
The privatization spells a major change for the long-established mail service.
Its familiar red pillar boxes will be protected by law, but tampering with a service so central to public life is also a delicate matter.
Even the late Margaret Thatcher, who championed the sale of state-owned companies such as British Telecom and British Gas, backed away from privatizing Royal Mail during her tenure as prime minister in the 1980s.
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