London Underground workers are to be balloted for industrial action in a dispute over training.
Members of the Rail Maritime and Transport union will vote on whether to mount a campaign of action short of a strike over claims that the company is using ill-trained and unqualified staff to make up minimum numbers at stations.
RMT general secretary Bob Crow said: "This move by London Underground puts passengers and staff at risk by placing the safe running of stations in the hands of minimally trained volunteers. It would be dangerous at any time, but especially during the period of unprecedented demand we are facing during the Olympics.
"It also breaches the settlement of our casualisation dispute in 2008 and totally undermines the role of station staff, threatening future staffing levels and job security.
"As a result we have no option but to move to a ballot and we hope that the anger and concern that this attack on safety-critical staffing levels has provoked will force LU to engage with us in meaningful talks."
Nigel Holness, LU's operations director, said: "All staff working in London Underground stations have had full and detailed safety training and work alongside qualified supervisors and managers. They are all qualified and are thoroughly assessed in order to receive their qualification.
"The RMT are well aware of our thorough approach to staff training and the use of incident customer service assistants, and we have a meeting scheduled next week to discuss their concerns."