A union is balloting members over strike action on the Woolwich Ferry amid allegations of "victimisation".
Unite announced the vote to take industrial action at the Thames crossing, which links North Woolwich and Woolwich, on April 7.
The ballot of 57 Unite members - which runs from April 14 to 29 - follows a claim by bosses that a union rep failed to deal with an oil spill on board, which he denies.
A disciplinary hearing on April 7 resulted in the worker being issued with a warning and the loss of his role as captain. He was accused of not motivating colleagues to deal with the spill, according to the union.
Unite said a second worker, who is also a member of Unite, has had his shifts changed so they clash with his duties as a carer.
Staff also allege there is a failure to agree a new pay and reward scheme; "excessive" use of agency staff; and "inadequate" health and safety training to new workers.
Transport for London (TfL) took over operation of the ferry from Briggs Marine Contractors, following disputes in 2017 and 2019.
Unite regional officer Onay Kasab said: "Unfortunately, we are experiencing a Groundhog Day scenario at the Woolwich Ferry.
"We welcomed the takeover by TfL. However, we are very disappointed that we are once again in the position of holding a ballot for strike action – we expected better of TfL management.
"No union can stand by while its representative is victimised.
"It is a fundamental principle that our members understand – when workers come forward to stand as union representatives, bravely putting their heads above the parapet, they will have the full support of the union, using every means at our disposal, including industrial action.
"However, we are keen to engage constructively with TfL management during the ballot process so we can resolve these outstanding issues and ensure the Woolwich Ferry can be operated in a fashion that truly benefits the users and the workforce."
A TfL spokesperson said: "We are working with Unite to resolve this issue as soon as possible."
Before the pandemic about 20,000 vehicles a week and 2.6million passengers a year were using the free service across the Thames, according to Unite.
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