Signal failures on London Underground (LU) have increased by almost half, according to figures from the mayor.
The most failures were on the District Line
Answers to written questions put to Ken Livingstone showed failures rose from 152 in January to 219 in April.
The Conservatives said Metronet and Tube Lines, the two firms responsible for maintaining the underground lines, had "some serious explaining to do".
Metronet said signal failures on its lines fell 4% over six months. Tube Lines said reliability had improved.
From 9 January to 30 April the most signal failures were on the District Line, with a total of 145, with April being the worst month, the Mayor's figures showed.
Tory transport spokesman Roger Evans said "a rapid and sizeable improvement in performance" was needed as failures caused "massive inconvenience".
"Last year they managed to significantly reduce the number of failures. Now they have shot back up again," he said.
New signalling system
"This suggests they have the ability to cut down on the number of failures but are choosing not to do so at the moment."
Metronet, which looks after nine of the 12 lines, said: "Signalling failures caused around 20% less disruption in our second year of operation as Metronet fixed faults faster, concentrating on locations where the public is most affected."
Tube Lines, which is responsible for the Northern, Jubilee and Piccadilly lines, said it had dedicated teams to tackle the problems on each line and was continuing to identify ways of improving reliability further.
It also said it would be installing an entirely new signalling system which would increase capacity as well as result in faster, more reliable journeys.