They will take action against rail operator One Rail for 24 hours on Saturday September 15 and again on Monday September 17.
One Rail guard Paul Yarwood was sacked following an incident with a violent passenger who was alleged to have fare-dodged.
“Our members across ‘One’ have shown their solidarity with Paul and their disgust at the company’s attitude to its own staff’s safety,” RMT general secretary Bob Crow.
More than 100 guards, revenue-protection inspectors and retail staff in north Essex have already taken two days of action in defence of Paul Yarwood, but now the strike has spread, with RMT members voting by more than five to one to join the dispute.
“It is astonishing that the company still seems to have its head buried in the sand, not least because of the huge public support that Paul has had from members of the public.
“Our members will be leafleting passengers during the strike to explain that the company’s bizarre stance has left us no option but to strike again, this time right across the company.
“The size of the strike vote should have told One everything they need to know about the effect their failure to support their staff is having on the people who have to face the daily danger of threats, abuse and assaults.
“I would like to thank all those who have already called on the company to re-instate Paul Yarwood, and would urge others to contact the company, or to let us have their comments so we can pass them on,” Crow said.
A One Rail spokesman said: "We believe we have comprehensively and satisfactorily responded to the concerns raised by the Union on safety and security and that industrial action is unnecessary. It should also be noted that less than 38% of the RMT's 805 members in One voted in favour of strike action."
One Rail expects disruption to the Norwich - London intercity services, and local services from Ipswich and Norwich across Suffolk, Norfolk and Cambridgeshire will be unlikely to operate. The Southend - London route may suffer some cancellations.