Speaking at the inauguration ceremony for the new Rosa Parks station, Valls said the "exceptional additional contribution" means construction can begin "now," although the opening date will not be brought forward.

The government has already agreed to commit €1bn to the project, which was confirmed by Valls in October 2014 and a further €1.75bn will come from the 2015-2020 State-Regional Contract (CPER), which was signed in June. However, in recent months talks between the project's backers have been deadlocked over how the remaining costs should be divided.

The so-called Eole project involves constructing an 8km tunnel from St Lazare to Nanterre-la-Folie and upgrading the existing 47km line to Poissy and Mantes-la-Jolie. The extension will cut journey times between western suburbs and La Défense, an important commercial centre on the west side of Paris, by up to 17 minutes.

It will also provide existing Line E passengers with direct access to La Défense, and will relieve congestion by between 10 and 15% on the Auber - La Défense section of RER Line A, which Line E will parallel, and part of RER Line C.

Major construction works will begin at La Défense later this year and the underground section from Hausmann St Lazare to Porte Maillot and Nanterre is due to open in 2020. The remainder of the line to Mante-le-Jolie is scheduled for commissioning in 2022.

The extension will interchange with RER lines A and C, metro Line 1, light rail Line T2, and Lines 15 and 18 of the orbital Grand Paris Express network. With the opening of the extension, Line E is expected to carry 89,000 passenger per hour in the morning peak and 620,000 passengers a day by 2022.

Eole map