Wessex Water is urging motorists travelling in the Chippenham area to allow extra time for their journey.
It follows the closure of the A4 Bath Road from 2pm today so emergency repairs can be carried out to a burst water main under the railway bridge.
The A4 will be closed between the junction with Hungerdown Lane and the junction which leads to Patterdown in both directions. Diversions will be in place.
A Wessex Water spokesman said: "Due to the location of the water main we have had to close the road to carry out the emergency repair work. We apologise for any delays this may cause to journeys.
"Due to quick action taken by Wessex Water staff customers were not left without water when the main burst at around 5am."
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