An RMT strike which is set to involve more than 50,000 railway workers is being backed by a second union.
Unite members who work for London Underground are set to join the RMT walkout on June 21, which is set to impact the railway network throughout London and the home counties - including Hertfordshire.
A Unite spokesperson said its strike would involve around 1,000 workers who are protesting a company plan which they say will devalue railway workers' pensions.
The existing strike involves staff at Network Rail and 13 railway firms - including five in Hertfordshire - on June 21, 23 and 25.
RMT and Unite members at London Underground will only strike on June 21.
Sharon Graham, Unite general secretary, said: "It is not acceptable in any way, shape or form that the dedicated workers at Transport for London and London Underground are being told to pay the price of the pandemic with their pensions, pay cuts and threats to their jobs.
"Unite’s members have their union’s total support. These unwarranted threats to our members’ retirement savings, pay and jobs must be taken off the table at once or strike action will hit the London’s transport network."
Mick Lynch, RMT general secretary, said: "Rail companies are making at least £500m a year in profits, whilst fat cat rail bosses have been paid millions during the Covid-19 pandemic."
He added: "We have a cost of living crisis, and it is unacceptable for railway workers to either lose their jobs or face another year of a pay freeze when inflation is at 11.1 percent and rising.
"RMT is open to meaningful negotiations with rail bosses and ministers, but they will need to come up with new proposals to prevent months of disruption on our railways."
In Hertfordshire, staff at Avanti West Coast, Chiltern Railways, Greater Anglia, LNER and West Midlands Trains are due to strike.
Staff at Great Northern, Southern and Thameslink are set to take "action short of a strike", although a lack of Network Rail workers could spell disruption on the network.
Some stations on Great Northern's route between Stevenage, Welwyn Garden City and London Moorgate are run by London Underground staff.
Passengers at Old Street and Highbury and Islington station will likely be impacted by delays or closures.
Elsewhere in the country, workers voted to strike at CrossCountry, c2c, East Midlands Railway, Great Western Railway, Northern, South Western Railway and Transpennine Express.
The London Underground strike on June 21 is different to the industrial action on Monday (June 6), which was about potential job losses among station crew.
Transport secretary and Welwyn Hatfield MP Grant Shapps said RMT's intention to hold a strike was "very disappointing".
Mr Shapps said: "It's very disappointing RMT Union is taking action which could damage the rail network after taxpayers contributed £16 billion - £600 per household - to keep jobs during Covid.
"We're working with the industry to reduce disruption caused by strike, but urge unions to come to talks with employers."
Ahead of the strike action on Monday, the CEO of London's Chamber of Commerce and Industry said the strikes are "detrimental" to people who commute to and work in London.
Richard Burge said: "The last two years hit London disproportionately hard and the capital is desperately trying to claw back some sense of normality after a tumultuous two years.
"This strike now puts TfL in a position of having to recommend that Londoners work from home.
"Ultimately, this will only harm London’s economy and it is time for TfL to sort out their dispute with the RMT so we can get back to building prosperity and showing the world that London is open business."
Andy Byford, TfL's chief operating officer, said: "It is disappointing that the RMT is threatening such disruption given that nobody has lost or will lose their jobs as part of the proposals that we have laid out, which amounts to a recruitment freeze rather than job losses, and that there have been no proposals to change pensions or conditions.
"The devastating impact of the pandemic on TfL finances has made a programme of change urgently necessary.
"I urge the RMT to work with us rather than continue to disrupt our customers and further hamper London's recovery.
"We have been in regular talks with the RMT to try and resolve this dispute and would welcome further talks rather than strike action."
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