Union leaders have disagreed with London Underground over the impact of a 48-hour strike by maintenance workers.
The action by the Rail, Maritime and Transport (RMT) union began at 1900 BST on Wednesday, in a row over proposed changes to jobs, pay and conditions.
Transport for London (TfL) said services were operating as normal despite the walkout on the Northern, Piccadilly and Jubilee lines.
The RMT said there was disruption due to signal faults not repaired.
BBC London's travel service said a normal service was running apart from some minor delays on the Piccadilly Line due to a signal failure.
"The RMT action is having no significant impact, and we intend to operate a full service throughout the duration of the strike," a TfL spokesperson said.
"All safety and emergency response duties are covered, and the network is operating as normal."
But the RMT said there were delays on the District, Piccadilly and Bakerloo lines because signal faults were left unrepaired and drivers were refusing to move trains due to safety reasons.
TfL denied the claims.
The strike went ahead after Tube Lines lost a High Court challenge on Wednesday to stop the strike.
The firm took the legal action after the RMT turned down a new three-year pay deal on Tuesday.
The firm had offered a 3.7% pay rise in the first year, backdated to April 2010, and another 0.5% subject to agreeing productivity changes.
The agreement over the next two years would have been in keeping with the retail price index (RPI) plus 0.5%, with 0.5% guaranteed if RPI falls below 0%.
Andrew Cleaves, acting chief executive of Tube Lines, said he was "deeply confused" at the rejection "when Aslef, TSSA and Unite have given strong indications that they are prepared to accept the same pay deal".
But Geoff Martin, spokesman for the RMT, said the deal was unacceptable and "gave no assurance on jobs, which is central to our dispute."