Concerns have been raised about the M27 bridge that runs over the River Hamble, between junctions 8 and 9, and how waste material coming off the bridge could destroy the river's ecosystem.
As part of the smart motorway work, National Highways reportedly agreed to solve drainage issues across the bridge, to ensure nothing falls into the river.
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Chairman of the River Hamble Harbour board, Cllr Sean Woodward, says the agency has 'tried to go back' on its word.
But National Highways insists it is still looking for a solution.
Cllr Woodward said: 'This issue has been a concern for decades now, because the drainage is literally just pipes through the bridge that lead straight into the River Hamble.
'The worst case scenario here would be that thousands of gallons of petrol or diesel could pour into the river in the event of an accident - it's an ecological disaster waiting to happen.
'We have to prepare a drainage system to ensure this doesn't happen, and we need to do that sooner rather than later.'
The River Hamble Harbour board is the decision-making body of the harbour authority, with a mix of politicians and marine representatives.
Main work on the M27 smart motorway project began in 2019, with a completion date of spring this year - at a cost of £244m, according to National Highways.
Cllr Woodward said: ‘National Highways agreed to the drainage works all the way back at the start, but then tried to go back on its plan.
‘They told us it was because everything was costing more than they estimated, so wanted to remove it from their plans.
‘The bottom line is that this remains a huge environmental concern for us - we haven't had a disaster yet, but we'd also like to keep it that way.
Greg Stone, National Highways M27 route manager, said: ‘We are aware of an issue with water run-off from the Hamble bridge, and it remains our intention to deliver improvements to the drainage in the area.
'We will continue to work with the Environment Agency to try and find the right solution to this issue.’