Travellers on London's Underground network face disruption as drivers belonging to the union Aslef stage a 24-hour strike.
The dispute is over the union's demand for extra pay for its members working on the public holiday.
London Underground says it will work to make sure as many services as possible will operate.
The strike is expected to affect shoppers heading for the start of the new year sales, and football fans.
Transport for London has warned of "severe disruption" and urged travellers to check its website before starting their journeys. At around 10:30 GMT the website showed all lines suffering some level of disruption.
It says it will lay on extra buses in busy shopping areas, with millions of shoppers expected to hit the High Street.
Arsenal have already moved their Premier League game with Wolves to 27 December because of the strike, but Chelsea's game with Fulham goes ahead at Stamford Bridge.
The BBC's Daniel Boettcher says Aslef has argued that choosing to work on Boxing Day should be voluntary and drivers who agree to do so should be paid extra as well as getting a day off in lieu.
If I did this for this trade union, then 18,000 other workers who've agreed the '92 agreement would have to be paid as well.”
End Quote Howard Collins London Underground
But Aslef general secretary Mick Whelan told the BBC News Channel that triple pay was "not quite what we've asked for" although it had been a negotiating point.
"What we're asking for is a guaranteed level of service where people can be guaranteed at least every other year, every third year, a day off, and - for those who do come in - an incentive." He said the nature of the incentive was still being negotiated.
"[London Underground] have picked on the worst aspect of a comment that was made and have used it continually throughout this process."
Mr Whelan said the original dispute focused on "quality time off". He said the union had realised an agreement it made in the mid-90s, when he said virtually no trains ran on Boxing Day had "become devalued" as the number of trains running on Boxing Day had increased to be the same as any other day of the week.
Referring to the strike's potential impact on London retailers opening for the Boxing Day sales, Mr Whelan said: "It's unfortunate that it happens at a time when we are in recession and possibly going into a double-dip. Truly, we'd like to see and help London business grow."
London Underground says it has a long-standing agreement with all of its trade unions covering bank holidays. It has described the demand for additional payment as outrageous.
Chief operating officer Howard Collins told BBC Radio 4 that London Underground had already met some of Aslef's concerns.
Selfridges Shoppers flood into Selfridges in Manchester for its Boxing Day sale
"We have reduced the number of train drivers working this year, giving people more quality time off - down to 880 drivers from 1,100 last year."
But Mr Collins said he could not "pay people twice" and that train drivers' salaries were all-inclusive and covered Boxing Day and Christmas Day.
If I did this for this trade union, then 18,000 other workers who've agreed the '92 agreement would have to be paid as well."
Labour has accused the coalition government of doing "nothing to encourage rail operators to run a Boxing Day service".
Shadow transport minister John Woodcock said the Conservatives had attacked the Boxing Day rail shutdown "year after year in opposition".
"Transport Secretary Justine Greening should tell us what she plans to do to ensure that Boxing Day services do run in future, or else admit that her party was simply chasing headlines in opposition by raising this issue, and had no real intention of doing anything about it," he said.
London Underground last week challenged Aslef's strike ballot in the High Court saying it was unlawful as the vote included drivers who were not rostered to work on Boxing Day.
But a High Court judge, Mr Justice Eder, said the disruption caused because of the strike was "regrettable" but the strike was "inevitable".
"I do not consider that the fact that the ballot included persons who would not themselves go on strike renders the ballot held in breach of (legislation)," he added.
If there there is no resolution to this dispute in the New Year, the union plans three more 24-hour strikes in January and February.
Aslef members plan to walk out again on 16 January, 3 February and 13 February over the same issues.
Services were severely disrupted on Boxing Day last year when drivers walked out over the "long-running" dispute.
Boxing Day sales
The strikes come as stores across the UK start their new year sales.
The BBC's business correspondent Emma Simpson says it has been a difficult year for many retailers, who have been discounting since November.
The breakdown service Green Flag estimated 5.6 million motorists would be on the road, with the shops one of the main destinations.
It forecast the Boxing Day rush to peak at 1030 GMT.
Argos, John Lewis and Marks & Spencer were among stores who started their sales online as early as Christmas Eve and many people are predicted to spend Boxing Day online rather than in shops.
E-retail association Interactive Media in Retail Group predicted on Christmas Eve that online consumers would spend £186.4m on Christmas Day and £367.8m on Boxing Day. Debenhams said it saw a 75% spike in Christmas Day online traffic compared with last year.