London Underground drivers have gone on strike on Boxing Day for the third consecutive year, causing major disruptions to Christmas bargain-hunters and visitors to the capital.
The train drivers' union Aslef stopped work today for the first of three strikes as part of a long-running dispute about Bank Holiday pay.
Two further walk-outs are scheduled for the last two Fridays in January.
With up to 7.1 million shoppers expected to hit the Boxing Day sales, Transport for London (TfL) said it is was doing everything possible to help shoppers get into and around London.
Services are operating today on most London Underground lines, however TfL is warning passengers of major disruptions and only limited services on all lines.
The Waterloo & City line is closed today, while other lines are operating reduced services.
The Piccadilly line is expected to be closed through the city centre, and the Victoria line is scheduled to run only between Seven Sisters and Victoria at a reduced frequency.
Extra buses will be laid on for those travelling to the West End, as well as the Westfield shopping centres in Stratford and White City.
London Overground services are not in operation today and the Congestion Charge has been suspended.
Some rail services are operating on Southern and South Eastern trains into London Victoria and London St Pancras International, as well as on the Gatwick Express and Stansted Express.
Aslef argues it is not to blame for today's industrial action, saying management has "sat on its hands and offered nothing constructive to resolve this dispute".
Over 90% of Aslef members voted in favour of launching the action for a third consecutive year.
But TfL has condemned Aslef for what it argues is a "completely unnecessary disruption to Londoners on Boxing Day".
Howard Collins, London Underground's chief operating officer, said: "Train drivers are paid a salary that reflects some Bank Holiday working, but the Aslef leadership is demanding to be paid twice for the same work and has rejected our attempts to resolve the matter.
"The scandalous actions of the Aslef leadership are an attempt to hold Londoners to ransom, and demonstrate a wholesale disregard for our customers - making life harder for shoppers, sports fans, retail workers and businesses amongst others at an important time.
"They also show a disregard for the thousands of transport staff who will be working hard to help people get around the capital."
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