Services in the Piccadilly line Tube serving Heathrow Airport have been suspended because of a strike by drivers.
Members of the Rail, Maritime and Transport union on the Piccadilly Line were "solidly" supporting a 24-hour walkout which ends at 9pm on Thursday night.
Members of the Aslef union were also on strike.
Passengers were advised to find alternative ways to get to Heathrow, which is busy with Easter travellers.
RMT congratulates our members for the rock-solid and determined support for today's action. They are making a stand against bullying, the ripping-up of procedures, and in defence of the safety of staff and passengers alike.
The blame for this action rests solely with the LU management who have walked away from numerous opportunities to resolve the issues at the heart of this dispute. It is time for LU on the Piccadilly Line to end the macho posturing and start talking seriously with our representatives.
The unions claim there has been a "comprehensive breakdown" in industrial relations.
This indefensible strike has been called to back drivers who refused to drive Piccadilly Line trains on their shifts - without good reason - resulting in delays to our customers.
London Underground operations director Pat Hansberry said: "There will be no service on the Piccadilly Line today due to 24-hour strike action, though other Tube lines will continue to run as normal. Our customers are advised to check our real-time travel information to help them get around."
A power supply fault will cause 'all day' disruption on the Central Line, Transport for London says.
Update: Severe delays due to a power supply fault. The disruption is likely to be all day, please use alternative routes where ever possible
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London Underground train drivers have voted by almost 9-1 to accept a pay deal for a new all-night Tube service, said Aslef.
Aslef said its members backed the four-year wage offer by 88%, clearing another hurdle in plans for a night Tube.
The service was due to start on some lines at weekends last September but has been delayed because of failure to reach agreement with the Tube unions over pay and conditions.
Aslef members on London Underground have voted by a huge majority to accept the improved offer on pay and night Tube.
Last year management tried to force through changes to rosters that would have meant unlimited increases in weekend and night shifts with a pay rise of just 0.5%.
Our members took two days of absolutely solid strike action to protect their working conditions and win an improved offer.
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