Eurostar and sister company Keolis – both controlled by French state-owned rail firm SNCF – are poised to win the contract to run trains on the London to Edinburgh route.
Shares in the two rival British bidders slumped amid market speculation over the imminent award of the deal to run the East Coast mainline service.
The value of First Group fell by £150 million, while Stagecoach, which has submitted a joint bid with Virgin Trains, saw £33 million wiped off its shares.
The Department for Transport is expected to announce the winning bidder for the East Coast line this week from the shortlist of three.
SNCF owns 55% of Eurostar, along with 40% by the UK government – which is being sold off – with the remaining shares owned by the Belgian government.