The Scottish Parliament voted against plans with Labour and Lib Dem MSPs backing a motion by the Greens against plans by Peel Energy for a plant at Hunterston.
However, minister of the ruling minority Scottish National Party can still ignore the motion and press ahead with the scheme.
The motion was voted for during a tense debate on climate change and the future of energy supply in Scotland.
Scottish Green Party leader, Patrick Harvie, said: “Parliament has last night made the most crucial decision about Scotland’s future since the passage of the Climate Change Act last year.
“This plant has been dogged by controversy from the start, and ministers now face a possible judicial review if they try and force it through.
“Let’s not waste everyone’s time … this project is going nowhere, and if they proceed they will be wasting taxpayers’ time and money as well as their own.”
Luke Walsh