The deal agreed between Aslef and Southern Railway to end a dispute over driver-only trains has been attacked as a "shocking betrayal" of workers and passengers.
Leaders of Aslef agreed to recommend the deal to almost 1,000 drivers after saying it would improve safety.
But the Rail, Maritime and Transport union (RMT) said after seeing details of the agreement that it would lead to the de-staffing of trains.
General secretary Mick Cash also lashed out at the TUC, which hosted 11 days of talks which did not involve the RMT.
He said: "This so-called agreement is a shocking and historical betrayal presided over by the TUC of 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 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."