PARIS (Reuters) - A showdown between French railway workers and President Emmanuel Macron’s government grew more threatening on Tuesday after an internal labor union message suggested rail workers could paralyze services even outside of official strike days.

Slideshow ( 3 images )

The message, attributed to a member of the hardline CGT union, urged colleagues to go beyond officially notified work stoppages announced for the coming months and cause network disruption on non-strike days as well.

“I’ve never seen anything like it, never seen a threat like it,” Guillaume Pepy, state-appointed chairman of the SNCF rail company, told RTL radio.

While the strikes by the SNCF’s rail workers are not scheduled to begin until next month, many of its staff have said they will join protests by public sector workers on Thursday in a show of solidarity against Macron’s reforms.

That is expected to halve high-speed and regional rail services, despite it not being an official strike day. The already announced SNCF strikes are scheduled to run for three months from April 3, with two days of action each week.

All of four main unions representing rail workers are angered by the government’s plans for change at highly indebted SNCF, including the abolition of job-for-life guarantees, automatic annual pay rises and generous early retirement.

Their protests are shaping up as perhaps the biggest challenge of Macron’s presidency since he took office last May on a promise to deliver a raft of economic reforms, some of which already appear to have had a positive impact on growth.

The CGT, the biggest union among the roughly 150,000 SNCF rail staff, spearheaded strikes in 1995 that forced the government of the time to withdraw reform, after which the then-prime minister, Alain Juppe, quit.

Opinion polls suggest Macron has substantial public support to take on the SNCF, with as many as three in four French people backing reforms. But unions have opted for a show of force and accuse his government of rushing through proposals.

The content of the email that SNCF chief Pepy cited first appeared in an overnight report by the daily Le Parisien.

“It has to be said that since 1995 we’ve taken one hiding after another,” says the message, urging workers to revolt. It advocates disruption of rail services by unspecified means on days other than the two per week already declared.

“Is this a call for acts of sabotage, damage to equipment? An explanation is required. I am extremely shocked,” Pepy said.

Two of the four unions representing rail workers have told members to join the day of public sector protests on Thursday, when a third of flights into Paris-area airports are expected to be canceled due to air traffic strikes. Schools will also be affected.

Thursday’s stoppages alone will halve regional rail services and standard inter-city links, and knock 60 percent of high-speed TGV trains out of action, the company said.