The Night Tube on one of London Underground's busiest lines will not run on Friday because of a continuing strike by workers in a row over industrial relations.
Members of the Rail, Maritime and Transport union (RMT) are taking action on the Piccadilly line, which serves Heathrow Airport.
Passengers have been suffering disruption since Wednesday, and services will not return to normal until Saturday morning.
Nigel Holness, Transport for London's director of network operations for London Underground, said: "I apologise to customers who are suffering three days of disruption for no good reason.
"The RMT unilaterally ended discussions at Acas on Tuesday and made no attempt to engage with us to try and resolve this dispute. Despite our request to meet the RMT at Acas they haven't yet agreed to talks. I encourage all customers to check before they travel, and reiterate that we remain available for talks with the RMT at any time."
RMT general scretary Mick Cash said: "RMT driver members stand united and determined this morning on day three of this strike in the fight for respect and justice in the workplace and a safe and supportive working environment on London's crucial Piccadilly line.
"It's now time for Tube bosses to recognise the anger and frustration of staff on the Piccadilly line and to start seriously addressing the deep-seated and long-running grievances at the heart of this dispute.
"The shutdown of the Piccadilly line service today and tonight is wholly the responsibility of an LU management who have sought out confrontation with their own workforce regardless of the impact on nearly 800,000 passengers a day on the main Tube artery to Heathrow Airport."