Labour MP Rushanara Ali has resigned from the front bench in order to abstain from voting on whether Britain should begin military action in Iraq.
The MP for Bethnal Green and Bow announced she would not take part in the House of Commons vote as she does not believe air strikes would be an effective in "targeting terrorists and not harming innocent civilians".
MPs voted to begin air strikes in Iraq to combat the militant group Isis (also known as Isil and Islamic State) by a majority of 481, with 524 voting yes and 43 voting no.
Ali, who was elected MP for Bethnal Green and Bow in 2010, said she was "unable in conscience to support the motion" and deliberately refrained from it.
In her resignation letter to Labour leader Ed Miliband, she said: "There can be no doubt that the actions of Isil are horrific and barbaric and I share the revulsion that everyone in our country feels towards them.
"However, I am not confident that this military action will be effective in the short term in just targeting the terrorists and not harming innocent civilians. Nor can I pretend to have any confidence that there is a credible long-term strategy to build up the capacity of the Iraqi army or that the potential impact on radicalisation in the UK has been properly thought through.
She added: "Despite good intentions, too many mistakes have been made over the last decade and far too many people in conflict zones have had to pay a high price for misconceived actions by the UK and other countries.
"However, there is a genuine belief in Muslim and non-Muslim communities that military action will only create further bloodshed and further pain for the people of Iraq."
In a reply letter, Miliband said Ali was someone with "great ability and talent", adding: "I respect your decision and accept your resignation."
Members of the Labour Party took to twitter to praise Ali following her resignation.
Speaking in the House of Commons, Prime Minister David Cameron said military action in Iraq was the only way to deal with the "psychopathic terrorists that are trying to kill us".
Cameron also warned defeating Isis could take "not just months but years" and assured it will not be a repeat of the 2003 invasion.
"The reason for that is that America, Britain and others are not contemplating putting combat troops on the ground," he said.
Cameron added all military decisions will be a result of "patience and persistence, not shock and awe" — a direct reference to the phrase associated with the invasion of Iraq.