A nine-day part-closure of London Bridge Station begins this Saturday, with no First Capital Connect or Southern trains until Monday 1 September.
The part-closure is the latest milestone in the rebuilding of the station as part of the Thameslink Programme.
"London Bridge is one of the country's busiest stations and we're rebuilding it piece by piece to transform north-south travel through London," said Nicky Hughes, Network Rail's head of communications for London and the South East.
"This is a huge challenge.
"There are alternative routes for everyone affected and we urge passengers across London and the south east to visit our new website, check if you are affected and then find out your alternative routes so you know your travel options."
Journey times between East Croydon and Blackfriars will be extended by up to 15 minutes.
Southern services will not operate to and from London Bridge and a revised service will be in operation across the network.
Southern services will not operate between No rwood Junction and London Bridge and between Crystal Palace and London Bridge.
Southern services via Peckham Rye will operate to and from South Bermondsey.
Some trains will be diverted to Victoria.
"I have met with representatives of Network Rail and Southern and made clear to them my view that they should have been doing more full scale communications and awareness raising work before now," warned Val Shawcross, London Assembly member for Lambeth and Southwark, last month.
"Thousands of commuters use these trains every day, and I am concerned that some passengers may have an unpleasant surprise when they discover that their trains are not going where they expect.
"I'm pleased that London Overground will be putting on extra services during the blockade and that passengers who usually use Southern to get to London Bridge directly and who will be directed to travel via Canada Water will have their tickets accepted on the Jubilee line.
"I would hope and expect that Network Rail and the relevant train companies will be pulling out all the stops so ensure no one is stranded during the bank holiday weekend and that everyone is able to plan an alternative route to work in plenty of time.
"I urge all users of London Bridge to make sure they are familiar with the new arrangements both during August's blockade and afterwards, when further service changes will take place and some trains diverted or cancelled. Be prepared and check your train today!"
Even when London Bridge reopens to Southern and First Capital Connect trains on 1 September, some trains will still be altered. A handful of rush hour trains will terminate at South Bermondsey rather than running all the way to London Bridge.
Although Southeastern services are not affected by this month's works, passengers on that company's trains will find their journeys altered from the start of next year.
Charing Cross trains won't stop at London Bridge from January 2015 until August 2016. Then Cannon Street trains will run non-stop through London Bridge until early 2018.
Southeastern recently published its draft timetable for 2015 incorporating the London Bridge changes.