Cabinet Secretary Arthur Sinodinos​ has refused to appear before a Senate inquiry into the controversial Free Enterprise Foundation and similar fundraising bodies, slamming it as a "political stunt".

But his refusal to appear has prompted questions about whether he is in contempt of the Senate.

Senator Sinodinos wrote to the committee conducting the inquiry on Wednesday night confirming his refusal to appear on Thursday, despite the Senate directing him to do so to answer questions.

In the letter, obtained by Fairfax Media, Senator Sinodinos describes the direction for him to appear as "both unprecedented and objectionable".