Cleaners at Waterloo Station are taking part in a ballot for strike action over claims of 'victimisation' and 'bullying'.
The cleaners are employed by Interserve and the RMT union is balloting its members between now and 4 November.
The union claims that its members have been victimised and underpaid, and subjected to racial abuse including one manager telling a member of staff that in their opinion "[Interserve] should not employ black people".
"The appalling treatment of cleaners at the busy and iconic Waterloo Station is nothing short of a disgrace and has led to a complete breakdown in industrial relations which leaves RMT with no option other than to ballot for strike action," said the union's general secretary Mick Cash.
"These staff are doing dirty jobs in often appalling conditions and the very least that they can expect is to be treated with dignity and respect.
"Not only are our members being denied the most basic levels of fair treatment and workplace rights but they are being subjected to a daily regime of bullying, victimisation and harassment which includes disgraceful incidents of racist abuse.
"Interserve cannot sweep this situation under the carpet. RMT is demanding workplace justice for the cleaners at Waterloo and the union remains available for talks aimed at securing that."
An Interserve spokesman said: "We are committed to providing a safe and supportive environment for all our staff and take allegations of this nature extremely seriously.
"We have been working with the union to investigate the issues raised.
"We are working with both staff and management, as well as their representatives and our client to resolve issues amicably and with the hope of averting industrial action."