The Rail Maritime and Transport union (RMT) is sending ballot papers to its 8,500 members to decide whether to go-ahead with a series of 24-hour, or even 48-hour, stoppages in September.

Talks between the union and London Underground failed after the company refused to go to independent, non-binding mediation.

The dispute concerns an initial pay offer from LU of 3.2% with a no-strike clause which the RMT turned down.

Later a 2.5% offer was increased to 3% which was also refused by the union.

But LU said it intends to bring in the rise anyway.

Last month three million people were affected by a stoppage over safety concerns.