London Underground (LU) workers are threatening to strike in a row over canteen and toilet facilities.
Some drivers have refused to book on at the Earl's Court depot since January in protest at the poor facilities.
The Rail Maritime and Transport (RMT) union said it would ballot its 9,500 members if disciplinary action is taken against drivers involved in the row.
LU said no-one had been threatened with dismissal but drivers were paid to book on at the west London depot.
'19th century conditions'
The spokesman added that millions had been spent upgrading staff facilities at Earl's Court and across the Underground network
He said: "We have an agreement with the unions and that discussions would continue to resolve the issue."
The RMT's Bob Crow said that any attempt to discipline drivers would be met with a ballot of the "entire London Underground membership".
"Our members were promised decent booking-on facilities, and here we are in the year 2005 and they're being threatened with the sack for refusing to accept 19th century conditions."