A report into infrastructure financing has advised against pouring money into "projects where the socio-economic viability is doubtful, the costs are unreasonable and the finance plan is still unknown".
Plans for LGV lines connecting Bordeaux with Toulouse and Dax, Poitiers with Limoges, and Montpellier with Perpignan were singled out as costly and unnecessary.
"We must focus on the renovation and modernization of the existing network," Marie-Hélène des Esgaulx, the committee's vice president, said. "Not doing so would be a mistake."
Senators recommended that the state and SNCF instead put an extra €1-2 billion into renovating the rail network; currently the budget is €2.5 billion each year.
The need to renovate the TGV lines was put into focus by a deadly crash in 2015, the first since the high speed lines went into service in 1981.
The senate is echoing several previous reports which have sounded the alarm about the state of France's rail network, however the new lines are still officially supported by the state and local authorities.
"At a time when public money is very scarce, major transport infrastructure projects should be selected with the utmost rigour," the senators wrote.
However, a €25 billion LGV line connecting Lyon with Turin in Italy would not be affected, due to "respect for our international commitments".