Conservative MP for Totnes Sarah Wollaston has rejected an offer of a junior government job.
Dr Wollaston said she was offered a job as a ministerial aide, known as a parliamentary private secretary (PPS).
But she said that would have stopped her voting against the government and she valued her independence as a backbench MP.
She said that becoming a PPS would have also prevented her tabling any amendments to health bills.
Dr Wollaston, who won the Totnes seat the at last year's general election, said: "I would have had to have left the Health Select Committee and I would never have been able to speak on any health matters.
"Even worse I would have always had to vote with the government and that's wrong.
"I would also not have been able to table any amendments and that's not something I would be prepared to do."
She said the rules governing MPs should be changed to allow more free speech among politicians.
She discovered about 150 out of 364 coalition MPs are on the so-called 'payroll vote', meaning that because of positions they hold, they have agreed to always vote with the government.
Included among those 150 are about 45 who work as a PPS.
She said: "Westminster is a fascinating place and a wonderful opportunity and I feel very privileged to be working here, but there are some things about how it operates that are immensely frustrating.
"I think they need to look closely at what they do and say 'Is this what the public want us to be doing?'"
Sarah Wollaston was interviewed for the BBC Politics Show South West, shown at 1200 on 20 February on BBC One.