Thousands of London Underground workers are to be balloted on strike action over pensions.
The ballot will end on 24 July
The Rail, Maritime and Transport Union opposes planned changes which it says could deprive workers of their pensions if they leave work owing to ill health.
The RMT says 11,000 of its members are to vote on whether to launch a series of strikes during the summer.
A Transport for London spokesperson said TfL had not been notified of any proposed ballot of its staff.
The spokesman said: "Any ballot for strike action is obviously premature as no decisions and no differences may exist at the end of the discussions.
"The way to resolve any differences that may arise is through the agreed procedures for discussion at this early stage."
RMT general secretary Bob Crow said: "This is a fundamental attack on a long-standing condition of service that would see most people who leave work through ill health deprived of their pension and we have made it clear that we will resist it.
"They backed away when we threatened strike action last year and these changes are just as unacceptable now as they were then."
He said a dispute could be avoided if TfL provided assurances it would abandon what Mr Crow described as "a cost-cutting attempt to undermine ill health benefits".
The ballot will end on 24 July, a day before a meeting in which a pension fund sub-committee will consider the suggested changes.