To elaborate on Frank II's post, from the TfL (Transport for London):
Transport for London (TfL) is advising customers that planned strike
action by the RMT union on four days from Tuesday 3 August until
Friday 6 August will cause severe disruption across the Tube network
if it goes ahead. The action would mean that all Tube lines would run
a significantly reduced service at some times and would not run at
others, and that customers should complete their journeys by 13:00 on
Tuesday 3 and Thursday 5 August. Further information will be available
ahead of, and during, the strike at tfl.gov.uk/tube-strike.