French rail operator SNCF said Tuesday that 90 percent of high-speed TGV trains across the country will not run on Thursday, when unions kick off an open-ended strike against a planned pension overhaul.

The SNCF expects 90 percent of regional trains as well as those on suburban lines in the Paris region will also be cancelled, while international services will be "severely disrupted".

"Thursday is going to be very difficult for our clients," SNCF spokeswoman Agnes Ogier said at a press conference. "We ask our clients to cancel their trips or postpone them if they are able."

Around half of Eurostar trains, including 16 between Paris and London on Thursday have already been cancelled, according to its website.

A third of the Thalys trains between Paris and Brussels will be cancelled, and just one round-trip Lyria TGV serving Switzerland will run.

The Paris metro is also bracing for gridlock, with 11 of the city's 16 lines completely shut Thursday.

Three other heavily used lines will operate at just one-third or one-fourth capacity -- and only during the morning and evening rush hours -- leaving just the network's two fully automated lines running.

For the two main suburban rail arteries crossing the city, the RER A and B, less than half the number of usual trains will run, again during rush hours only.

Thursday is expected to be the most severe public transport strike since a massive protest last spring against an SNCF overhaul Thursday is expected to be the most severe public transport strike since a massive protest last spring against an SNCF overhaul Photo: AFP / Christophe SIMON

France's DGAC civil aviation authority warned that 20 percent of French flights will be cancelled Thursday as air controllers and others walk off the job.

Flag carrier Air France said it had cancelled 30 percent of domestic flights for Thursday, and 15 percent of mid-range flights serving destinations outside the country.

If it goes ahead, it would be the most severe public transport strike since a massive protest last spring against an overhaul at the SNCF -- which President Emmanuel Macron eventually succeeded in passing through parliament.

The SNCF said it has chartered buses to replace the trains on the most heavily used lines, though it warned they will not have the capacity to take on every ticket holder.

The SNCF had already taken the unusual step of halting ticket sales from Thursday through Sunday ahead of the strike, which unions have vowed to maintain until Macron abandons the pension reform.

The government says its pensions overhaul, one of Macron's campaign promises from 2017, is necessary to keep the system financially viable as the population ages.

But unions say the plan for a "universal" system will require millions of workers in both the public and private sectors to work longer or face severe cuts to their pensions.