Passengers from the Brighton and Hove area can expect more delays on Thameslink services today (Monday 26 January).
The tunnel between Farringdon and St Pancras International is still flooded after a water main burst at the end of last week.
Thames Water said that it had been working round the clock to try to tackle the problem.
Network Rail is keeping the water levels as low as possible with a pump but there is still a significant flood.
A spokesman for Govia Thameslink Railway, which operates Thameslink services, said: “A burst 16in water main will continue to affect our services until Thames Water can fix it.
“It’s safe for us to run our trains through the water as long as Network Rail can keep the flood levels down to a manageable level with pumps but we have to run a reduced service.”
The company plans to operate a reduced service today, with two Thameslink trains an hour running between Brighton and London – half the usual number.
Thames Water said: “We’re really sorry to commuters who have had disruptions to their journeys.
“We have been working around the clock this weekend, with help from Thameslink, Network Rail and Transport for London, to fix the leak as fast as possible while keeping our customers in supply at the same time.”