Passengers on the Northern, Jubilee and Piccadilly Tube lines had to contend with a sharp increase in signal failures this summer, new figures show.
The data released by Tube Lines revealed signal failures rose by almost 50% from 22 August to 18 September compared with the same period in 2003.
The firm said its best quarter-year performance since it took over the lines was between July and September.
It added that reliability targets were met on all three lines.
A Tube Lines spokeswoman said: "We've made significant steps in the design of a new signalling system for the Jubilee and Northern Lines, completed improvements to 50 Piccadilly Line trains and have teams on site modernising 15 stations.
"There is still a lot of work to do to deliver an outstanding Tube for London and we share other passengers' disappointment when problems result in delays.
"Huge levels of investment we're injecting into the infrastructure are gradually leading to more reliable journeys in a better environment."
Tube Lines is responsible for investing in and maintaining the Jubilee, Northern, and Piccadilly lines.