A fresh wave of industrial action will be held this week amid the bitter dispute over the role of train guards.
Members of the Rail, Maritime and Transport union (RMT) at South Western Railway (SWR) will refuse to work any rest days from Friday for four days.
The union, which is embroiled in similar disputes at four other train operators, said the action will affect services.
Officials accused SWR of sending “threatening and intimidating” letters to union members suggesting they will be making “illegal” deductions to the wages of staff to recoup losses arising from the current industrial dispute.
RMT general secretary Mick Cash said: “It is frankly disgraceful that South Western Railway point-blank refuse to engage with the union in meaningful talks over their plans to run trains without a safety-critical guard on board across this franchise.
“Instead, they have resorted to the crudest possible attempts to threaten, bully and harass our members fighting to put public safety before private profit.
“South Western Railway should call off these disgraceful attacks on their front-line staff and should start talking seriously with the union around an agreement that underpins the guard guarantee and ensures safe, accessible and secure services for all.
“It is the continuing intransigent attitude of the company which has forced us to put on this latest phase of industrial action in an effort to force them to see sense and to drive them back to the negotiating table for genuine and meaningful talks.
“We know that this action will have a serious impact on services and the responsibility for the disruption caused will be wholly down to South Western Railway and their pig-headed attitude. It is time for the company to get out of the bunker, stop threatening their staff and start talking.”
A South Western Railway spokesman said: “What our letter refers to is if guards refuse to work other elements of their contracted terms and conditions, which were jointly agreed with the union several years ago as part of a restructuring package, and allow for some flexibility around shift start and finish times.
“By telling their members to refuse to work flexibly they are putting their members in breach of their contract. We have advised the union that we consider this to be strike action and will respond accordingly.
“Should this action go ahead, we will do everything we can to minimise the effect on passengers and will aim to run our full service. However, we sincerely hope the union withdraws this action and does not put its members in this impossible position.”