Rail passengers planning a Christmas getaway face severe disruption due to engineering work on several lines.
Network Rail has warned some routes will have "significant changes to normal services" and urged people to plan their journeys as early as possible.
But some timetables will only be confirmed six weeks in advance, instead of the standard 12.
It follows delayed enhancement projects and the launch of new trains and services after "unprecedented" investment.
Some of the festive improvement work will begin on December 23 - the last working day before Christmas - to give engineers "three clear days to make uninterrupted progress".
Network Rail said there will be less overall disruption than during the 2017 festive period as most of the engineering work has been scheduled for when no trains are planned to run, such as on Christmas Day and Boxing Day.
A number of lines serving London's major stations and airports will be blocked.
Major work is expected to affect London Paddington between December 23 and December 31.
From December 23 to 25 and on December 30 there will be no Heathrow Express services, while Great Western Railway trains will terminate at Slough or Reading.
Passengers are urged to avoid Clapham Junction station from December 23 to January 1 as it will be exceptionally busy due to platforms nine to 19 at London Victoria being closed.
No Gatwick Express trains will run, while most Southern services will be diverted to and from London Bridge.
Engineering work is taking place at various points around Manchester Victoria, closing lines during the Christmas and New Year period.
Track improvements between Bedford and Kettering as part of the Midland Main Line Upgrade will also affect timetables from December 24 to January 1.
Andy Thomas, managing director of strategic operations at Network Rail, said: "This Christmas and New Year, thousands of rail workers will be working round the clock to deliver crucial upgrades to the rail network as part of the £50 billion railway upgrade plan.
"This huge investment programme will deliver more reliable infrastructure, improved facilities and services for passengers when they return to work.
"We know that our railway is up to 50% quieter than usual during the festive period so taking on and delivering these huge transformational schemes at this time of year minimises our impact on passengers who, research shows, understand the need for such activity.
"While most of the network is open for business as usual, some routes are heavily affected and so we strongly advise passengers to plan ahead."