Christian Taxil, EDF board member for managers' union CFE-CGC, wrote to employees saying his decision was due to ongoing concerns about the project.
"Today, conditions are not right for me to give a positive opinion if such a project were presented to me," Christian Taxil, EDF board member for managers' union CFE-CGC, said.
Taxil, one of six union representatives who sit on EDF's board, cited current energy prices, technical issues with the nuclear reactors it plans to build and the company's weak balance sheet.
He also said the development of renewable energy sources, which have priority access to the grid, mean EDF could struggle to sell all the power that the nuclear reactors eventually produce.
However, EDF's board is still likely to approve the deal when it votes in early May, with sources telling Reuters only six out of 18 board members would vote against it.
Earlier today, EDF was forced to defend Hinkley, after an internal report written by two engineers calling for a delay of at least two years and a reactor design was leaked to the Financial Times.
In a statement emailed to Reuters today, EDF bemoaned "unfounded rumours and fantasy information" in the media without being any more specific.
"EDF denies these rumours (and) confirms that the start-up date for the first reactor is fixed for 2025 and that no delay is envisaged," EDF said.