This comes as ASLEF, which represents train drivers, announced it would ballot its members next week over the possibility of further strikes.
The RMT union, which represents conductors, has been locked in a year-long dispute over driver-only operation with Govia Thameslink (GTR), the company operating the Southern franchise.
It said its members will strike on Monday, July 10; it would be the 32nd day of strike action.
Mick Cash, RMT general secretary, said: “RMT members on Southern Rail have been fighting for safety and access for well over a year now.
“The franchise continues to lurch from crisis to crisis as the real impact of hacking back on safety and access hits home. We have seen absolute chaos at Victoria and East Croydon this week in the hot weather which has reinforced the need for safety critical staff on our trains and platforms. The Government cannot spin their way out of this chaotic situation which is wholly of their making.
“It is now down to Southern/GTR, and the contract holders in the minority Government, to face up to their responsibilities and engage in genuine and serious talks that address our issues.”
A spokesperson for Southern said: “Once again the RMT is refusing to play its part in the modernisation of the railway. Southern is the most congested network in the UK with passenger numbers into London doubling in just 12 years, so modernisation of infrastructure, trains and working practices is essential to provide the service levels passengers need and deserve.
“While we have been willing to negotiate and have made four comprehensive offers, the RMT has not demonstrated any serious intent to resolve its dispute. It has not put any of our four offers to its members, including our latest of May 16, to which we’ve had no formal response. This offered guarantees on job security, staffing ratios, rostering and enhanced training.
“The RMT is relying on an outdated 14-month-old ballot for this action. There have been very significant changes in the situation so it no longer provides a mandate for strike action. We can only speculate about the true reasons for the RMT’s actions and its desire to continue making passengers suffer.”