The rebels tabled a hard-hitting amendment to the Transport Bill, which has its second reading in the Commons next week, warning that safety would be put at risk by the partial sell-off of National Air Traffic Services (Nats).
The motion says that "government ownership and control of Nats is necessary for national security" and expresses concern that "the introduction of the private profit motive could jeopardise safety standards in the future".
The Labour MPs argue that Britain's air traffic control system is recognised internationally as the safest and most reliable in the world and describe the Bill as "not acceptable" because of the partial sell-off plan.
Although the amendment will not be put to a vote in next week's two-day debate, it was seen as a powerful warning shot that ministers will face the biggest backbench revolt since the general election unless they make concessions.
The rebels hope to persuade John Prescott, the Deputy Prime Minister, to water down his plans during the Bill's passage through Parliament. If he refuses, they will seek to defeat the measure when it has its third reading.
One rebel said last night: "If you did a poll of the Parliamentary Labour Party, you would struggle to find 20 per cent of the MPs supporting this Bill."
The 50 signatories do not include others with grave doubts about the plan, including Gwynneth Dunwoody, chairman of the Transport Select Committee and Martin Salter, MP for Reading West.
Mr Salter said he would vote against the Government for the first time if no compromise was reached, but was hopeful that a deal could be done.Reuse content