A large hole on the A27 at Titchfield is now being repaired, but work will take several days to finish.
The hole, which measures about a metre long and 50cm wide, opened up in the inside lane of Southampton Road, heading away from Fareham.
Drivers have been urged to take alternative routes.
Workers from Hampshire County Council’s highways department are investigating.
The council’s executive member for transport Sean Woodward said: ‘Bridge engineers have been to the site this afternoon and organised a diver to assess the extent of the damage.
‘We now know that of the four culverts that carry the River Meon under the A27, three appear to be intact and one of the 600mm pipes looks like it has collapsed.
‘We are currently making arrangements for floodlights and trench supports to be delivered to site and the intention is to work right through the night to excavate the collapsed section of carriageway and expose the full extent of the problem.
‘We will then review things early tomorrow morning when we will be in a better position to identify the best technical solution.
‘The likelihood is that the inside lane will need to remain closed for the next few days but a lot will depend on what we find when we start digging.’