The organisation faces a "fundamental threat" from email and must be reformed in order to survive, the Liberal Democrat insisted.
But unions have warned that services will go into decline if the business is sold off.
Dave Ward, deputy general secretary of the Communication Workers Union, whose members will be balloted on the proposals this week, claimed loopholes would allow the universal service obligation to be eroded.
"There are opportunities for them to down dial the terms of the universal service in the future," he told BBC Radio 4's Today programme. "People who live in the suburbs, people who live in rural areas, people who live in villages will not get the same service under a private company."
It comes as Royal Mail is expected to announce results that show a significant increase in profits.
Mr Cable told the programme: "There is no alternative. The point is that it is not this particular government adopting an ideological position.
"We are following in the footsteps of the last government that recognised that certain things had to happen in order for Royal Mail to survive because it is faced with a fundamental threat to its business from email."
The National Federation of SubPostmasters said the plans should be postponed to prevent an "even deeper crisis" for Britain's post offices. It is concerned that a privatised Royal Mail would not feel obligated to keep services with the Post Office network.