The Circle Line is subject to severe delays this afternoon due to a signal failure at Gloucester Road station.
There is no service between Aldgate and Edgware Road in the clockwise direction, with severe delays on the rest of the line.
Transport for London said tickets will be accepted on London buses within zone 1 only.
The District Line has also been hit by the signalling problems, with minor delays between Earls Court and Ealing Broadway/Richmond.
This afternoon's disruption follows a morning of chaos across the capital's Tube network, which saw delays on the Central, Northern and Piccadilly Lines.
The Piccadilly Line is still severely disrupted, due to the fact that half its fleet is out of service for repairs at the moment.