Mayoral candidate Jon Rogers has spoken out against plans for a £30 billion barrage across the Severn Estuary.
Mr Rogers is worried it would jeopardise future growth at Avonmouth and Portbury docks which provide jobs for thousands of workers in the Bristol area.
Dr Rogers, the Liberal Democrat candidate, has decided to speak out because the barrage schemenow seems to be back on the political agenda.
David Cameron made positive comments about the scheme during the summer and the Welsh MP Peter Hain stood down as shadow Welsh Secretary several months ago to concentrate on making the barrage happen.
Dr Rogers (pictured) said: "The latest talk about a barrage is causing potential difficulties for jobs and investment opportunities at the port.
"A report was published in 2010 which said a barrage would cost too much for what it would gain and therefore everyone was far more relaxed about future investment in the port. But if it were to go ahead, then it could have a very negative impact on the port which I want to see protected.
"I am very strongly in favour of the port and of course, I want to see future investment to protect existing jobs and create new ones."
He said there were many other ways of harnessing tidal power to create "green" energy instead of building a barrage.
Among these are tidal fences, tidal reefs and lagoons which are much cheaper and cause much less environmental damage.
He said these would be able to generate electricity throughout a 24-hour period compared with a barrage that could only operate four or five hours a day.
Bristol is fast becoming a centre for firms which specialise in new technology to create renewable energy.
He said if the barrage was scrapped in favour of alternatives, then these firms could develop and export their expertise worldwide.
He said if a barrage was built, then it would have a dramatic effect on the tides which scour the estuary bed to create a deepwater channel for some of the world's largest ships.
If the estuary and the entrance to the docks began to silt up, then it would have a massive impact on the future of the port.
The Bristol Port Company, which was formed in 1991 when it bought the port from the city council, directly employs about 600 staff.
But scores of firms, both big and small and which rely on the port, employ a total of nearly 8,000 workers in the Bristol area.
They range from crane drivers and lorry drivers to tug operators and warehouse staff.
The ten-mile tidal barrage which would stretch from Somerset to South Wales would generate five per cent of Britain's electricity needs and claims have been made that it would create thousands of jobs.
Port spokeswoman Sue Turner said: "We are always talking to potential investors and they always have to weigh up any risks involved.
"Anything that slows down ships is a potential risk. Delay is a bad thing.
"Even talking about the barrage can damage potential investment prospects.
"There is nothing that a barrage would do to generate jobs in Bristol – it would damage job prospects."