More than 100 cleaners working for support services firm OCS on its Eurostar contract are voting on strike action.
The ballot follows a row over pay between the company and the RMT union, which represents the cleaners.
The ballot of members, who are being to be asked to vote to strike and for action short of strike, will close on 21 November.
The union is campaigning for a minimum pay rate that matches London mayor Ken Livingstone’s recommendation of a living wage of at least £7.05 an hour.
“OCS has been styled as a family firm, yet it is paying our members wages that no-one could hope to feed and clothe a family on,” RMT general secretary Bob Crow said.
“Cleaners on as little as £5.50 an hour – even for nights, Sundays and bank-holiday work – are being offered a rise that works out at 20p an hour.”