Highways Agency engineers have confirmed the 15ft opening is a Dene Hole.

Dene Holes are underground chalk caves built by the Danes across Kent and Essex in the Middle Ages.

The motorway was partially closed following the collapse and contractors poured 40 tonnes of aggregate into the hole on Wednesday.

The Agency said: “Geotechnical investigations are continuing to find the cause to allow the repair to be safely completed so that the remaining lanes can be reopened to traffic.”