Michael Portillo is standing down from the board of BAE Systems over concerns that his media work may pose a conflict of interest.
Mr Portillo stood down as a Tory MP at the last election
The ex-Tory defence minister said he was quitting the £45,000-a-year post because of his need to speak freely on any topic.
BAE said Mr Portillo believed there was a danger that people would presume his views were shared by the company.
BAE Systems is the UK's biggest defence contractor and makes the Eurofighter.
In the past Mr Portillo, who joined BAE in 2002, has voiced his thoughts on controversial topics such as nuclear power.
He called for the UK to scrap nuclear missiles because the collapse of the Soviet Union meant they were no longer necessary and said the money could be better spent elsewhere.
In his letter of resignation, Mr Portillo said there was a danger that his views "might be thought to represent the position of the company, or alternatively, be thought to be at variance with the company's views".
"Having considered the problem carefully he believes that it can only be resolved by standing down from the board," BAE said in a statement.
Mr Portillo is a regular political commentator on the BBC's This Week programme and also pens a column for The Sunday Times.