Last-ditch talks are being held in a bid to avert a strike on the London Underground line serving Heathrow airport.
Members of the Rail, Maritime and Transport union on the Piccadilly Line are due to launch a 48-hour walk-out from noon on Wednesday and for five hours from 8.30pm on Friday.
The union says there has been a complete breakdown of industrial relations and Transport for London warned there will be little or no service if the strike goes ahead, including the Night Tube on Friday.
The two sides will meet at the conciliation service Acas on Monday.
RMT general secretary Mick Cash said: “This dispute is about a comprehensive breakdown in industrial relations over a series of long-running unresolved grievances including the abuse of procedures and failure to implement key safety and operational improvements agreed in the past with the union.
“RMT will be attending the talks looking for serious progress.
“Our arrangements for the strike action starting Wednesday are in place and the ball is firmly in London Underground’s court.”
TfL has advised Piccadilly line passengers to complete journeys by 1pm on Wednesday if the strike goes ahead.
Nigel Holness, TfL’s director of network operations for London Underground, said: “We will provide customers with the latest information in order to help them plan their journeys during this possible strike action.
“The TfL website, journey planner and social media channels will all feature up-to-date travel advice and will be updated throughout the week.
“We have met with the RMT leadership to identify areas where we can work together to resolve this dispute.
“I urge them to continue to work constructively with us so that Londoners aren’t subjected to disruption across several days.”