Two days of rail strikes have been called by Southern staff over the introduction of driver-only operation.
The RMT union has held a number of walkouts since April last year in its battle with rail operator Govia Thameslink Railway.
Its members have been transferred from the role of guards to on-board supervisors, with responsibility for opening and closing doors given to drivers.
Today it confirmed strike action on both Friday September 1 and Monday September 4.
Mick Cash, RMT general secretary, said: “RMT is bitterly disappointed that Southern Rail have rejected our call for round table discussions involving all parties with an interest in resolving this dispute.
“The failure to get those talks moving following our face to face meeting with Transport Secretary Chris Grayling has left us no option but to call further action.
“RMT pays tribute to our Southern Rail guards and drivers who have stood firm for well over a year in the fight for rail safety.”
He added: “They are a credit to their communities and to the whole trade union movement.
“RMT has a clear plan for resolving this dispute but that requires round table talks now to push forwards. This fiasco cannot be allowed to drag on any longer.
“The union stands ready and waiting for genuine and meaningful talks with all parties and we are pressing Mr Grayling to get his contractors into line as a matter of urgency.”
But a spokesman for Southern Railway said: “We are disappointed by this unnecessary action and the RMT’s refusal to engage with us in modernising the railway. We deeply regret any inconvenience this action will cause our passengers and we will make every attempt to minimise disruption.
“Modernisation is urgently required to future-proof and increase capacity on the busiest parts of the UK railway.
“This modernisation needs three things: investment in better infrastructure, new trains, and changes in working practices. The first two elements are being rolled out but the Trade Unions need to play their part if passengers are to benefit from service improvements.”