Councillor Samantha Davies has said new procedures to renominate Conservative councillors for election were the catalyst for her decision to leave the party.
In a statement, she said: "The Brentford and Isleworth Conservative Association has recently changed the rules regarding the re-nomination of Conservative candidates for the local elections in 2010.
"Basically, it is a form of mandatory reselection' and I have been reliably informed (and believe them) that this was a way of dealing with awkward' sitting Conservative councillors."
"I am not prepared to leave myself open to being deselected in such a fashion (not that I have actually voted against a whipped vote in the council chamber - I have only expressed dissenting views privately) and have therefore decided that I will sit as a Chiswick Independent'."
Coun Davies expressed concern the new procedures could limit the ability of councillors to speak out against the Conservative/community group-led executive on issues such as parking enforcement.
But the Conservative leader of the Council, Peter Thompson, denied there were any plans to scrap the committees.
He said he was "surprised to read in Coun Davies' resignation statement that she has concerns about certain policy areas" as her opposition to reselection procedures was the only reason she had given the party for her departure.
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