The company behind the controversial scheme, Helius Energy, were due to put in a planning application before the end of the year.
But today it has been revealed that proposals will not be submitted until early next year.
Helius Planning Director Paul Brighton said: “We will now be submitting in the first quarter of 2013.
“We’re taking longer to consider the public consultation responses.
“It’s taking time to put all the documents together under the procedures that we need to go through.
“There’s an enormous amount of documentation that needs to be prepared.”
The development comes just a month after the firm confirmed the chosen design for the £300 million plant.
Three options were put to the public during the summer, and the company says that 52 per cent of respondents chose the “marine” look.
But campaigners said the consultation was flawed.
They dismissed the statistic – saying that many people put “none of the above” on the design consultation and these results have not been included.
Campaigners against the plans met the news of the delay with a mixture of anger and relief.
No to Southampton Biomass spokesman Steven Galton said: “I am disappointed because the residents are always the last to hear of these developments.
“There’s nothing on the Helius website and they haven’t responded to anyone.
“I’m not even holding my breath that the application will be submitted in the new year because I think it will drag on and on.
“But the longer it drags on, the more research there is that biomass isn’t actually environmentally friendly or green.
“More and more people are looking at the details in the scientific community and the longer it takes the more research will come through that biomass is not the answer.”