Hundreds of commuters from Brighton and Hove will face having to use a different London station over the next six weeks.
Blackfriars closes tomorrow until the end of the year as part of the Thameslink upgrade.
Most commuters from Brighton and Hove travel to London Victoria, London Bridge or East Croydon.
But the work at Blackfriars is expected to disrupt the journey to and from work for a significant number of commuters.
The station is undergoing a “realignment” which is intended to make it easier to accommodate longer trains.
It is one of a series of upgrades being carried out by Network Rail to try to tackle overcrowding.
