Govia Thameslink Railway (GTR) workers will join the national rail dispute after an emphatic ballot result.
GTR members were originally balloted as part of the national rail dispute along with 13 other train operating companies in May.
However due to the anti-trade union laws, despite voting yes, there was insufficient numbers to allow them to legally take part in strike action last week.
Following a turnout of 57.5pc and a huge yes vote of 80pc, over 2,000 GTR staff will now take part in the next strike days if and when RMT decides to announce further stoppages.
The result comes hot on the heels of fellow rail unions Aslef and TSSA announcing that they will ballot their members for strike action on the railways.
If the Aslef ballot is successful it will be the first time train drivers have had a national strike since 1995.
RMT general secretary Mick Lynch: "This fantastic result is a credit to our elected officers and lay activists who worked around the clock to deliver this emphatic result.
"Our members were denied their democratic right to strike due to draconian ballot thresholds but having smashed through those at the second time of asking, they are raring to join our campaign.
"GTR staff are now able to join our fight and have a legal mandate to take action in the future, to get a negotiated settlement on pay, job security and working conditions.
"We will continue our negotiations next week with Network Rail and the train operating companies with a view to getting a deal that meets our members a just deal.
"We cannot rule out further strikes and we are focused on winning this dispute and coming to an agreement with rail bosses that gives our members what they need."
On the news that TSSA and Aslef balloting their members, Mr Lynch said: “We welcome Aslef and TSSA balloting their members for strike action. If rail unions stay united in our common struggle for pay justice, job security and decent working conditions, we cannot and will not be defeated.”