A burst main on the A468 near Bedwas Rugby Club is affecting water supplies to homes and businesses across the Caerphilly town area.
A team of engineers have been at the scene since around 2.40pm undertaking repairs.
The water main, which is under the A468 Newport Road, has burst sideways underground through the pavement.
Water is gushing out onto the road and flooding Bedwas’ rugby pitch and a nearby field.
The water is fresh water and not sewage. However, the flood may put Bedwas’ crunch relegation match against Swansea on the weekend in doubt.
Bill Price, a Bedwas RFC director, said: “At the moment, the clubhouse is okay, but it is all according how long the repair will take.
“We’ve got a big game this weekend – the last one of the season.”
Mr Price said that the main drains running along the side of the road are blocked and is making the situation worse.
There is also concern from engineers at the scene that the road could be weakened because of the water.
A Welsh Water spokesperson said: “We are aware of a burst on our water main on Bedwas Road next to Bedwas Rugby Club. This could be affecting supplies to some of our customers in the Caerphilly and Bedwas areas where people may be experiencing low water pressure or have no water.
“We have a team on site to isolate the burst and carry out the necessary repair. For the work to be done safely, we need to have temporary traffic lights in place while the work is being carried out.
“We are very sorry for the inconvenience this is causing and assure everybody affected that we are doing all we can to restore normal service as quickly as possible.
“As supplies begin to return, some customers may notice a slight discolouration of their tap water. This is not unusual after an incident of this nature and should clear within a short period of time.
“If anyone has concerns about their water, we would ask they call our operational helpline on 0800 052 0130.”
Earlier this morning, a burst pipe caused similar problems in the Cefn Fforest and Blackwood areas.