The strike on Southern Rail has been called off to allow for fresh negotiations to take place.
Acas said that the strike had been called off but Southern said services could still be affected tomorrow.
"Southern and the RMT are returning to talks tomorrow under the auspices of Acas. Strike action has been suspended for Thursday and Friday," Acas said in a statement.
"We are encouraged that the RMT has accepted our offer to resume talks at ACAS and has agreed to call off its strike action. For our passengers sake we truly hope these talks will be productive and bring this long running dispute to an end," Southern said.
"At present, the strike timetable is still in the industry train planning systems for Thursday and Friday. Regrettably, this means tomorrow’s service will be based upon the present strike timetable but we will do our very best to add services in and extend the hours of operation wherever possible. On Friday we plan to revert to the revised timetable operating before the strike."
Southern said it will update its website as talks continue.
RMT general secretary Mick Cash said in a message to members: "The National Executive Committee has had time to consider this matter and have acknowledged that some progress is being made.
"As the company have now agreed to meet with our union officials without the caveat of any preconditions the NEC has therefore instructed me to inform you and your colleagues that the strike action has now been suspended with immediate effect until further notice," Cash added, according to Sky.
Thousands of commuters have had their journeys disrupted this week as a five-day strike called by the RMT hit Southern, which serves London Bridge and Victoria. Prime Minister Theresa May condemned the strike, saying: "It’s only going to cause more disruption and misery for passengers."
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.