LONDON—London’s transport operator warned customers on Thursday that strike action next week may go ahead causing disruption across the underground network. The walkout is planned between Aug. 24 and Aug. 27 and would mean a significantly reduced service on all lines for most of the week, and that journeys may need to be completed early, Transport for London said. The first of two 24-hour walkouts is due to start on Tuesday at midday and run until one minute before midday on Wednesday. The second will begin at midday on Thursday and run until one minute before midday on Friday. Passenger numbers on the underground are at less than half their pre-pandemic level, putting financial pressure on TfL which has needed multiple bailouts from the government over the last 18 months. The Rail, Maritime, and Transport union is calling the strike over changes to London’s night tube service. The separate pool …