As the prime minister prepares to set out his plan for tackling the crisis in Syria and bolster support for air strikes against Islamic State, shadow chancellor John McDonnell has said the issue has to be approached in a "very considered way".
He told the BBC's The Andrew Marr Show the shadow cabinet will consider the issue of going to war carefully.
"This is the most serious thing you can do as you're affecting people's lives and they could lose their lives".
Mr McDonnell added that he hoped Parliament "can act as one, put aside party differences and look at the long-term interests of the country".
"I think we've got to come at this in a very considered way on the basis of the experience we've had so far", Mr McDonnell continued.
"We have to be clever in the way we address this", Mr McDonnell added, "on the ground Isil needs to be defeated by the regional powers within the Middle East itself and the people".