Crispin Blunt has quit as a Tory whip because he wants to openly support Sir Malcolm Rifkind as next party leader.
Mr Blunt has quit the frontbench before
Sir Malcolm said earlier this week it was "quite likely" he would be a candidate to replace Michael Howard.
Mr Blunt said he could not continue in his frontbench job without affecting the "neutrality of the office".
In 2003 Mr Blunt tried to trigger a leadership contest against Iain Duncan Smith quitting a frontbench job as soon as polls closed in the local elections.
However the results were better than predicted and Mr Duncan Smith survived nearly six more months before being ousted by Conservative MPs.
Names in the frame?
Michael Howard then took over the reins for 18 months up to the General Election - but then announced his intention to stand down before Christmas this year.
A number of names are in the frame to succeed Mr Howard but Sir Malcolm, back in Parliament after an eight year gap, is the only one to say he is likely to be a candidate.
Mr Blunt released a statement saying that in view of the probability of Sir Malcolm's candidature, he felt compelled to step down.
TORY LEADERSHIP RUNNERS AND RIDERS:
Sir Malcolm Rifkind
He said Sir Malcolm was the "best equipped" candidate to be next Tory premier.
"Following my four years as Malcolm Rifkind's special adviser when he was foreign secretary and defence secretary my personal and professional respect for Sir Malcolm is well known," he said.
"In any leadership contest it is necessary that the Whips' Office remains publicly neutral regardless of the private views of individual members of that Office.
"Given my views and what are correctly assumed to be my views about the merits of Malcolm Rifkind as a potential leader of the Conservative Party I do not feel I could continue as an Opposition Whip without compromising the neutrality of the Office."
On Tuesday Sir Malcolm said it would foolish to pretend he was not interested in leading the Conservatives.
He has said it is an "not an option, but a necessity" for the party to win back the centre ground.
Mr Howard did not quit immediately because he wanted the rules governing the selection of a leader to be changed first.
Among those also thought to be considering standing for leader are Kenneth Clarke, David Davis, Liam Fox, Alan Duncan, Tim Yeo and David Cameron.