The void, of at least 140 cubic metres, extends beyond the area cut into the road surface.
Road users intending to travel north or southbound on the A1 near Newcastle and Gateshead are advised to expect delays as urgent work continues.
The hole is at least 3m deep and around 6.5m diameter and is believed to be related to old mine workings in the area.
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Rob Beckitt, duty operations manager at Highways England, said: "Safety is our top priority; we have to ensure the carriageway is totally safe before drivers use it.
"We and our contractors have been working hard with partners since the hole was discovered last night, and will continue to do so to make the carriageway safe as quickly as possible.
"In the meantime I urge all drivers in the area to check conditions before they set out and to leave plenty of extra time for their journeys. I would also like to thank drivers in advance for their patience as we carry out this complex task."
(CD/JP)
