Earlier this year, concerns were raised about a U-turn on plans to solve drainage issues along the M27 bridge that runs over the River Hamble.
As part of the smart motorways work, National Highways reportedly agreed to solve the drainage issues, which left untouched could cause environmental problems for the river.
Concerns were then raised by the River Hamble Harbour board that the agency was going back on its word - but National Highways insisted it was still looking for a solution.
Now, the Department for Transport (DfT) has confirmed that this drainage work will take place, but won't be completed until 2025.
Baroness Charlotte Vere of Norbiton, under-secretary of state in DfT, wrote a letter to River Hamble Harbour management committee chairman, Cllr Sean Woodward, confirming the decision.
She said: 'I understand that National Highways attempted to develop a solution to the drainage issue at Hamble Brige, but that this unfortunately could not be incorporated as part of the current M27 junctions 4-11 smart motorway scheme.
'However, National Highways' Designated Funds programme has a scheme for the Hamble Brige planned for feasibility in 2022/23, design in 2023/24 and construction in 2024/25, subject to securing approval of funding for each stage.
'National Highways will engage with Hampshire County Council during the fund scheme development and scheme works information will be provided to all appropriate authorities.'
The M27 smart motorway project, which has now been completed, cost around £244m, according to National Highways.
Cllr Woodward has welcomed the news - but fears an ecological incident could still take place in the meantime.
He said: ‘It has been delay after delay with this drainage issue, and we have been banging on about this for almost 20 years.
‘If there was some sort of spillage it would go unintercepted into the River Hamble, into sensitive habitats that we are trying to protect.
‘We were promised it as part of the smart motorway project and then it didn't happen, which was disappointing - I'm pleased there's now been another commitment to this work but to have to wait another few years simply isn’t acceptable.’