London Underground (LU) workers have voted in favour of strikes in a row over pensions.
The union wants assurances over the ill health pensions
The Rail Maritime and Transport union said its members had backed industrial action by 3,141 votes to 212.
The union said the proposals would "dramatically affect" the pensions of workers who leave work because of ill-health.
Transport for London (TfL) said the ballot was unreasonable as no decision on the pension has been made.
The union is in dispute with a number of Tube employers, including LU, TfL and Metronet, over proposed changes.
The union said the changes would mean anyone leaving their job through ill-health, who was capable of earning an income, would probably not qualify for a pension.
RMT general secretary Bob Crow said: "We do not want to have to bat these changes away time after time, and we are seeking guarantees from all the employers involved that they will not seek or back cost-cutting changes that will undermine ill-health benefits."
A TfL spokesman said: "The RMT ballot for industrial action is totally unreasonable as no decisions have been taken to change existing ill-health pension arrangements.
"It is therefore difficult to see what industrial action could achieve.
"We have written to the RMT making clear that any changes to the Transport for London pension scheme would be made by the pensions fund trustees, who have a broad representative membership including the unions. They have not made any changes."