LU power control staff are on strike. From the BBC:
The long-running dispute involves workers who transferred to other organisations years ago coming back under London Underground management, but with different working conditions.
The strike began yesterday evening and is due to run for 8 days until 19.59 on 9 July. LU says that they are willing to continue negotiations to attempt to find a solution that satisfies all parties. They expect no impact on Tube running.