Shadow trade and industry secretary David Willetts has indicated the Tories will vote against any move to transfer a majority stake in the state-owned firm to its staff.
Speaking to the Financial Times, Willetts said the plans, which would need MPs' approval, would break a Labour election promise to keep the firm in public hands.
There is already widespread discontent among Labour MPs about the move, with a rebellion likely.
The leadership of powerful postal union the Communication Workers Union is also opposed to the move. Under their current deal, postal workers already benefit from a bonus scheme, which paid out around 拢1,000 each earlier this month.
The part-privitisation plan has been put to ministers by Royal Mail chairman Allan Leighton. He is proposing that the government, which owns 100% of Royal Mail, lend it 拢2bn to buy shares over three years. The cash would go towards reducing its 拢2.5bn pension deficit.
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