Communications union CWU slammed the Con-Dem coalition manifesto published on Thursday as "old politics wrapped in new language."
Prime Minister David Cameron and his deputy Nick Clegg agreed a five-year programme of "partnership" government as they revealed the full details of the coalition deal at the Treasury.
The agreement covers 31 key areas from banking and defence to transport, health and education.
It will also seek to ensure "an injection of private capital into the Royal Mail including opportunities for employee ownership," but outlined that Post Office Limited would remain publicly owned.
The CWU accused the coalition government of pushing ahead with privatisation while breaking up the Royal Mail Group.
"This is old politics wrapped in new language," said CWU general secretary Billy Hayes.
"The British public has consistently rejected the privatisation of Royal Mail."
Mr Clegg told the audience of civil servants at the Treasury: "Even if you've read 100 party manifestos, you've never read a document like this one."
Yet the manifesto programme bore similarities to Labour's in its call for the creation of a green investment bank to fund energy schemes as proposed by former chancellor Alastair Darling in March.
It also sets out to establish a high speed rail network, as already unveiled by former transport secretary Lord Adonis, and supports Crossrail.
A commitment to continue with the renewal of Trident and the war in Afghanistan are again highlighted.
Mr Hayes added: "The move to regurgitate failed policies will be deeply unpopular."
Labour was forced to shelve plans for part-privatisation of Royal Mail under the guise of "modernisation" after the CWU's massive strike last autumn.
SNP postal affairs spokesman Mike Weir MP added: "Even part-privatisation would spell the beginning of the end for Royal Mail and open the door to job losses, service cuts and a deterioration in the working conditions of postal workers."
The cross-party Campaign for Public Ownership also hit out at the new government's plans to part-privatise Royal Mail, allow privately owned businesses to run state schools and its decision to allow profiteering train companies longer franchises.
NASUWT teaching union leader Chris Keates said: "The section on schools continues to ignore the views of the public. So much for empowering communities and evidence-based policy making we have been promised by the Conservatives."
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