One of London's Night Tube lines is in "crisis" because of a lack of drivers, a union has claimed.
Only seven trains ran on the Piccadilly line last weekend instead of the normal 22, the Rail, Maritime and Transport (RMT) union said.
It warned the situation may last until Christmas.
London Underground (LU) admitted some drivers were off due to sickness but the service had been a success since it launched last August.
RMT general secretary Mick Cash said: "Tube bosses need to send out a clear signal that they are addressing the driver shortage issues which have reduced the Night Tube on the Piccadilly Line to chaos."
Twenty-four hour services operate on the Central, Victoria, Jubilee, Piccadilly and Northern lines every Friday and Saturday.
Brian Woodhead, LU's operations director for the Piccadilly line, said: "I apologise to customers who were unable to use Night Tube services on the Piccadilly line last weekend because some train drivers were absent due to sickness.
"The Night Tube has been a real success in supporting London's night time economy and we are fully focused on running a reliable service on the Piccadilly line."