The deal agreed between Aslef and Southern Railway to end a dispute which has crippled London and the south of England has been attacked as a "shocking betrayal" of workers and passengers.
Aslef, which represents train drivers, and the RMT, which represents conductors and others workers on the network, were in dispute with Southern's parent company, GTR Southern, over its plan to introduce driver-only trains.
Earlier this week, Aslef and GTR Southern reached an agreement, pending a ballot, which averted strikes, but now that the RMT has had sight of the agreement it has said it would lead to de-staffing of trains.
General Secretary Mick Cash told PA: "This so-called agreement is a shocking and historical betrayal presided over by the TUC, not only the conductor grade and drivers, but also passengers, including disabled passengers, who have lost the guarantee of a second member of staff on their trains.
"This abysmal document lists a whole host of areas where a train can leave without a second member of staff that will leave both the driver and passengers exposed and vulnerable, and which also represents a thin end of the wedge that will lead to the de-staffing of trains."
Mr Cash added: "Loyal and dedicated conductors, who have fought for safety for over a year, have had the legs kicked from under them by those who are supposed to be on their side.
"Passengers and staff alike have been sold out by a stitch-up cooked up in Congress House by the TUC and the bosses whilst Southern, one of the most anti-union and hated companies of recent times, is laughing all the way to the bank and have been given a free run to rip up the safety rule book in the name of profits.
"This is not a deal, it is a disgrace, and the RMT dispute remains on and we will fight this shocking betrayal with every tool at our disposal."
RMT and GTR Southern are to hold talks next week to resolve their long-running row. Aslef's ballot result is due on 16 February.