Drivers in east and south-east London could face misery on the roads in 2017 if Woolwich Ferry workers opt for strikes.
Trade union Unite said it would ballot its 36 members working for Briggs Marine Contractors on industrial action over “failure to abide by agreements, health and safety concerns, and a culture bullying and sexual harassment”.
The company operates the service on behalf of TfL .
Unite said its ballot would take place between Tuesday, January 3 and Friday, January 13 and it expects fellow union GMB to also ballot its members.
Unite regional officer Onay Kasab said: “There is a very unpleasant culture of bullying and harassment at this company, including serious allegations of a sexual nature made by one of our female members.
“Our members have also reported that health and safety is being compromised through lack of training and that the bosses are trying to cut costs by failing to abide by agreements.
“If this strike goes ahead, thousands of passengers will suffer disruption and have to use alternative routes.
“The Woolwich Ferry, which has been in use since the 14th century, remains a major transport link for the capital.
“It needs to be made crystal clear that any industrial action would not have a political agenda.
"It would be a democratic and legitimate fight back against a poor employer and appalling working practices.”
About two million passengers and drivers use the ferry to cross the Thames each year.
Sadiq Khan has pledged to purchase new boats to make the service more reliable.
A similar ballot took place for staff on the service last year, when Unite also invited Briggs Marine to "get round the table".
A spokeswoman from Briggs Marine said: “Briggs Marine Contractors has today received notice of this ballot from Unite, after writing to the union only last week requesting talks to resolve its concerns.
"We do not recognise the issues set out in Unite's press statement and we have again requested talks with the unions to seek resolution.
"Most importantly, we will do everything we can to avoid inconvenience to the public."
TfL has been contacted for a statement but has yet to respond.
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