Bus workers are to stage nine weeks of continuous strike action amid an ongoing pay row.
First Cymru services to and from Llantwit Major have already been affected by two walkouts in the past fortnight.
On Monday, the Unite union announced it would step up its industrial action well into New Year - with an ongoing strike running from Thursday 20th November to Wednesday 21st January.
The operator has been accused of refusing to negotiate "in a reasonable manner" but First Cymru says it's in the hands of the union to stop what it called "unnecessary action".
Unite general secretary Sharon Graham said: "First Cymru is trying to take industrial relations back to the dark ages with its refusal to pay monies owed, attempts at union-busting to get staff to cross picket lines and all the while paying some of the lowest wages in the industry."
"Unite never stands for such behaviour. First needs to think again about how it is treating its workforce."
The first four-day strike began on 30th October after members rejected a fresh pay offer from First Cymru, followed by a further walkout, which ran from Bonfire Night until last Saturday.
Routes across South and West Wales were affected, including services linking Llantwit Major with Cardiff, Bridgend and Talbot Green.
According to Unite, First Cymru pays drivers £13.40 an hour, compared to £14.44 an hour for Stagecoach drivers in South Wales and more than £15 an hour for Cardiff Bus workers.
The current dispute involves workers based at First Cymru's depots in Swansea, Port Talbot, Bridgend, Carmarthen, Haverfordwest and Ammanford.
But the managing director of First Bus in Wales has pledged to keep "as many services running as possible".
Doug Claringbold said: "We, like our customers, are deeply frustrated that the union has announced two full months of strikes in south and west Wales from November 20 and continuing right through to January 21."
"These are unnecessary and, most importantly, they will be hugely disruptive to our customers during the festive period when many hope to use buses to shop, meet friends and family and enjoy Christmas parties. It is within the hands of the union to stop this action."
"We have always maintained an open dialogue with the union and while they continue to reject our offers we still hope to find a solution to encourage the union to call off their strikes."
"We have been committed to improving the pay of drivers, and alongside our above inflation offer, we continue to invest in staff facilities and conditions, and a range of benefits, including healthcare schemes."
Unite argues that drivers are asking for a reasonable rate of pay taking into account both inflation and the current cost of living.
Alan McCarthy, Unite's regional officer, added: "The pressures of the role are clear. Driving a bus is a highly skilled job."
"Yet drivers for First Cymru are treated like second class citizens and have reached the end of their tether. They are struggling to make ends meet and put food on the table for their families. Unite will be backing them every step of the way in this dispute."
"The communities of South Wales have been let down by First, who see fit to short change their drivers and will now see the consequences of this as our members take to the picket line."
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