The RMT 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" pay rise.
RMT general secretary Bob Crow said: "The era of poverty pay for cleaners must come to an end."
Initial Facilities said it was "continuing talks" with RMT.
OCS is yet to comment.
Initial Facilities said: "We are continuing talks with the RMT as well as re-examining our contingency plans to ensure that services to the general public are uninterrupted in the event of the union pursuing strike action."
Mr Crow said: "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."
Workers involved in the pay row clean trains at Ebbsfleet International and London St Pancras stations.
Meanwhile 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 would be major disruption across the Belgian rail network on Monday, with services between Brussels and London unable to operate for 24 hours from 22:00 GMT on Sunday.
Passengers were urged to travel before or after the strike, which will also disrupt flights.