Passengers at Clapham Junction and Wimbledon stations could be left queuing 'for hours' in disruption caused by a massive revamp at Waterloo.
The London terminal is undergoing refurbishment from August 5 to 28 as part of £800m works which will see four platforms extended, making room for longer trains, as well as the laying of new track.
Some stations, such as Earlsfield and Norbiton, will be completely closed while others will have a reduced service. The peak evening time route from London Waterloo to Wimbledon will be slashed to less than half their current provision.
Commuters have been warned that some journeys may take longer and some stations will have queuing systems outside.
A 'well-placed' source told the Sunday Times at the weekend that it could lead to delays and overcrowding.
South West Trains has issued advice to people to consider alternative routes and prepare for queues.
A spokesman told this newspaper: "There will be temporary changes to services during this time, with fewer trains running. As a result, some stations will be much busier and there may be queues to enter or leave some stations.
"We have been working with Network Rail since August 2016 to give passengers as much notice as possible and encourage everyone to plan their journey and consider alternative routes or ways of travelling if needed."
During the three weeks in August when the work is carried out, South West Trains said that from 8am to 8.59am, there will be 12 services to London Waterloo from Wimbledon, six less than the usual 18 services.
There will be 22 services from Clapham Junction, 11 less than usual.
At peak evening time, from 5.30pm to 6.29pm, there will be just 8 services from London Waterloo to Wimbledon, which is ten less than there are currently.
There will be 28 services from Waterloo to Clapham Junction, down from 36 during the normal period.
South London stations such as Earlsfield and Norbiton will be completely closed along with the offshoot of stations from Chessington South to Malden Manor, with a reduced service running from the next station, Motspur Park.
Charing Cross, London Bridge and Waterloo East stations will also be shut from August 26 to September 2, covering the bank holiday weekend and, for many, the first week back at work after the summer holiday.
Canon Street station will be closed on the weekend of August 26 to 27.
On its website, South West Trains say they will be running longer trains, temporarily opening the former international terminals at Waterloo, working around the clock and providing information in advance.
A Network Rail spokesperson said: "We are continuing to proceed with our plans for the August partial closure of Waterloo. We have no plans to further reduce services beyond that which has already been widely communicated to passengers."
Passengers can go to southwesttrains.co.uk/wswupgrade or search 'Waterloo Upgrade' online and select their station from the drop down list to get detailed travel advice for their journey and understand how the changes will affect them