Govia Thameslink Railway (GTR) has vowed to move ahead with its modernisation plans after talks with RMT ended without any agreement.
GTR, the parent company of embattled Southern Rail, said it will move forward after talks broke down with the RMT union at Acas.
An Acas spokesperson said: "Acas conciliation talks have ended without the sides reaching agreement. There are no further talks planned but our services remain available."
Angie Doll, GTR passenger services director, said: "We have been talking to the union for nine months now and, despite several visits to Acas, the union won't agree a deal.
"Passengers will be rightly exasperated that the RMT won’t agree to what most fair-minded people would believe is an incredibly good offer. We are guaranteeing jobs, pay and a second person on as many trains as we do today and also offered to work with the RMT to agree modern working practices to reduce cancellations and passenger disruption.
The RMT has repeatedly tried to play the safety card as the issue but it did not raise this issue at all during these latest talks, confirming this dispute is purely about union power and control. The fact is that, day in, day out for decades, up and down Britain’s railways and the Tube network, we’ve had the driver operating the doors, safely.
This is backed up by independent research and expert opinion, including that of the Rail Safety and Standards Board.
Subsequently, GTR said it will "now move forward with our modernisation plans which will deliver better customer service for our passengers".
However, it added that its eight-point proposal is still on the table and it urged the RMT to give this serious consideration.
RMT general secretary Mick Cash said: "We had a golden opportunity in these talks to make some serious progress on the core issue of a second person on the train who would have protected the safety of passengers, delivered customer service and ensured access to services for those with disabilities or needing assistance.
"It's a bitter blow that a firm set of union proposals that could have allowed us to move forward were rejected out of hand. The matter will be discussed by the union executive this afternoon."
The news comes after a strike by the RMT on a number of lines last week, including on Southern.
The decision by Southern to transfer control of doors to the driver – a concept familiar to millions of London Underground commuters – triggered the long-running dispute, which has seen staff sickness levels soar. Controversially, only 393 guards were balloted on the strike action.