Cleaners on London Underground are to be balloted for strikes in disputes over pay, pensions and benefits.
The Rail Maritime and Transport union (RMT) said its members employed by contractors ISS and Initial will vote in the coming weeks on whether to launch a campaign of industrial action.
General secretary Bob Crow said: "Once again RMT is forced to move to a ballot for industrial action to secure a fair deal for our cleaner members and to end the exploitation of these groups of staff who work round the clock in the most appalling conditions to keep the Tube running.
"In the run-up to the Olympics, it is simply scandalous that the staff who will be keeping the transport system clean for the millions of visitors to this city are being treated like dirt."
The announcement was made as RMT members who maintain the Northern, Jubilee and Piccadilly lines continued with a strike over pensions.
They walked out at 4pm yesterday and will remain on strike until the same time on Friday.
Severe delays were reported between Morden and Kennington on the Northern line today due to a signal failure.