Major train service changes will affect thousands of train journeys during three years of disruption.

Passengers are being urged to plan alternative routes if they need to travel to London Bridge from Brighton or other Sussex stations from December 20, with an estimated 38,000 journeys expected to be affected.

There will be 16 days when Southern Railway, Thameslink or Southeastern trains will not stop at London Bridge while engineers work to open two new platforms and modernise 1970s track and signalling equipment.

From January 5, there will be some reduced services until the next step of work to expand the 178-year-old station is completed in 2018.

From January 12, no cross-London Thameslink trains will call at the station but a reduced service will operate from Brighton and terminate there.

There will be only two trains in the three-hour morning peak period and two off-peak trains an hour between Monday and Saturday.

Until August 2016, Southeastern’s Charing Cross services will not call at London Bridge and some services will be diverted to Cannon Street while engineers work on new tracks and platforms.

Southern trains will operate as normal. 

The £6.5 billion government-sponsored Thameslink building programme is expected to increase passenger capacity by 65% and provide a concourse at London Bridge bigger than the pitch at Wembley Stadium.

Simon Blanchflower, Thameslink programme director, said: “In the long run our work here will improve the journeys of millions of passengers across the South, but we understand until we’ve finished some people may have to change their journey to or from London.”

In a joint statement the companies said: “We’re working hard together to make sure our passengers get the best travel advice and help during this massive project. We’ll have more staff out to help and lots of information.”

During the Christmas period, all National Rail tickets will be accepted on “reasonable” routes across its networks and on London overground, underground and buses, South West Trains, and the Docklands Light Railway services.

From January until August 2016, tickets issued by the affected companies will be accepted at limited South London underground stations, with no extra charge. According to Thameslink the stations included are:

West Hampstead (Jubilee Line), Kentish Town, King's Cross St Pancras, Farringdon, Blackfriars, Borough, Elephant and Castle, Southwark, London Bridge, Bank / Monument, Cannon Street, Canada Water, Victoria, Waterloo, Embankment, Charing Cross, Whitechapel, Balham, Wimbledon.

Season ticket holders have been contacted and some will be issued with new passes by January 12, a Thameslink spokesman said.

Visit thameslinkprogramme.co.uksouthernrailway.comthameslinkrailway.com, or southeasternrailway.com for more information.