The Unite union is set to ballot its members on London Underground for industrial action over pay.
The ballot of engineering, electrical, administrative and management workers will coincide with a ballot of RMT transport union members on the tube over the same issue.
London Underground has refused to move on its two-year pay offer – 1.5 percent in the first year and the RPI rate of inflation plus 0.5 percent in the second year.
RMT members struck over pay and other issues in June. A yes vote in both ballots would likely see any action being co-ordinated by the unions.
Drivers in the Aslef union have also rejected the pay deal, but it is believed that the union will not ballot for action until next year.