Workers who clean Eurostar trains and stations on the UK side of the route have voted to go on strike in a row over pay.
The Rail Maritime and Transport union said 150 of its members working on three separate contracts for OCS and Initial earned little more than the national minimum wage and had been offered a "derisory" increase.
General secretary Bob Crow said: "Our members have spoken with one voice on all three of these contracts, and it should be quite clear to OCS and Initial, as well as Eurostar and Network Rail, that the era of poverty pay for cleaners must come to an end.
"The offers so far on the table would all amount to a real-terms pay cut and are way below what our low-paid members need to secure a fair and just income, and way below the standard set by the London Living Wage.
"Both contractors have made substantial profits on the backs of low-paid staff who do a difficult and crucial job in keeping the Eurostar moving, and we are calling on them to return to the table for serious talks, or face the prospect of strike action.
"Eurostar made more than £92 million in profits last year, way ahead of its own target, and they and Network Rail have a moral obligation to ensure that workers on the Channel Tunnel network do not suffer this kind of super-exploitation."
Workers involved in the pay row clean trains as well as London St Pancras and Ebbsfleet International stations.
* Passengers planning to travel by rail to Brussels next week have been urged to change their plans because of a national strike in Belgium.
Eurostar said there will be major disruption across the Belgian rail network on Monday, with services between Brussels and London unable to operate for 24 hours from 10pm on Sunday.
Passengers were urged to travel before or after the strike, which will also disrupt flights.