Conservative whip John Randall has quit his post because of his concerns over a possible Iraq war.
Randall wants more freedom to speak his mind on Iraq
With the Tory leadership backing Tony Blair's stance on the Iraq crisis, Mr Randall's resignation shows divisions are not exclusively confined to the Labour benches.
His move comes after International Development Secretary Clare Short launched a searing attack on Mr Blair's "reckless" Iraq policy.
One Labour ministerial aide, Andy Reed, has already resigned over his concerns and others have signalled they will follow if war begins without new United Nations backing.
Uxbridge MP Mr Randall said: "I wanted the freedom of the
back benches to be able to express my views about Iraq.
"I do not think at this
stage military action is justified.
"I was, in fact, against war in Kosovo when people like Clare Short were all
for bombing the hell out of it."
Mr Randall entered Parliament in a by-election in 1997 and was made a whip in 2000.
He actually resigned on Wednesday but thought it would be unhelpful to announce his decision until now.
"Mr Duncan Smith, whose leadership I support, said he was sorry to lose me but added that he always understood when people had matters of principle like this," added Mr Randall.
The 47-year-old MP is the first opposition figure to resign over the Iraq crisis.
But 13 Tory MPs, including former Chancellor Ken Clarke, did vote with Labour rebels against the government in the latest Iraq debate.