A road in Watford has closed amid fears a a sinkhole exists deep under the surface, after a gap appeared in St John's Road.
Preliminary investigations carried out last week had to be stopped as there were concerns about undermining the road and footway further.
Hertfordshire County Council said: "A geotechnical engineer present during the investigations advised that there is a possibility that a sinkhole exists deep down in the underlying chalk."
Following further investigations on Monday, the road is being shut until work on it can be completed. Investigations will include excavating a hole from the pavement to the middle of the road.
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It is hoped the hole can be filled
The council added: "It is possible that by exposing the hole and then subsequently back-filling, this will cure the problem. If this is the case, the road is likely to be closed for up to four days. A small section will be resurfaced upon completion.
Terry Douris, the council's cabinet member for highways, said: "I am aware of the importance of this road and we apologise for any inconvenience that these urgent works may cause residents and road users.
"It is vital that this work is carried out without delay to ensure everyone's safety. We will do our very best to keep disruption to a minimum, but we are aware of the impact that these roadworks will inevitably have."