Labour's annual conference is set to discuss a call not to support Britain remaining in the European Union if employment rights are watered down as part of any membership renegotiations.
The TUC has already warned that workers will not support a Europe that fails to protect their rights, safety, and conditions such as working hours.
The GMB union has now submitted a motion to Labour's conference later this month attacking any moves to "undermine" rights, raising the issue of whether the party should refuse to support a vote to remain in the EU in the forthcoming referendum.
The motion reads: "Conference deplores the actions of the Prime Minister, touring EU capitals and attempting to seek the agreement of other EU Member State governments to rob British workers of the vital social and employment rights they have gained from Europe, making them third class citizens of the EU.
"Conference calls on the Labour Party to insist that David Cameron and his Government present to Parliament a detailed account of the objectives of their reform negotiations in advance of addressing the European Parliament.
"Conference furthermore confirms that the Labour Party will resist any attempts by the Government to scale back the application of EU employment and social rights to British workers in this process, and that if EU Member State governments allow Cameron to remove these rights, then the Labour Party should not support a vote to remain in the EU."
GMB leader Sir Paul Kenny told the TUC Congress in Brighton: "The Prime Minister of the UK has been poncing around Europe seeking an agreement to cut the rights of working people in Britain.
"It is shameful that a British prime minister should be prostituting himself for the pimps at the CBI, begging other European heads of government to make British workers the second class citizens of the European world of work.
"We know existing EU protections are being eroded but this latest government sponsored attack on the whole tenet of social Europe brings those of us previously in the yes to Europe camp to the edge of fundamental change.
"The balance has gone, now a naked attempt by this government to remove fundamental rights will force us to ask ourselves a difficult for some question."
Other so-called contemporary motions from unions to Labour's annual conference, also being held in Brighton, will include criticism of the Trade Union Bill.