Business secretary Lord Mandelson is working towards the sale of a minority stake in Royal Mail to a private operator, which is expected to be TNT, under plans revealed in December.
The rebellion against the move has been spearheaded by Geraldine Smith, the MP for Morecambe and Lunsdale, who tabled an early day motion in Parliament today.
The motion has been signed by 66 Labour MPs, according to the BBC, or 71 Labour MPs, according to Sky News. It states the plans "risk fracturing one of Britain's greatest public services".
But the MPs' opposition is unlikely to stop the government's plans as the main opposition parties are generally supportive. The Tories have described the part-privatisation as a "step in the right direction".
Mandelson has told the BBC the government intends to push ahead with the plans.