Nick Clegg has described how he is "wrestling" with the decision of whether to support military action in Syria.
The Deputy Prime Minister said doing nothing about Bashar Assad's regime in Damascus was "taking a very active choice".
He told BBC Radio 4's Today programme: "A terrible war crime has been committed a week ago. We face a judgment, a choice, everybody does by the way, every single MP.
"I'm struggling with this, I'm wrestling with this."
He added: "These are no easy choices to make and there is no risk-free choice to make."
The Liberal Democrat leader said if the situation in Syria deteriorated further, then "entirely separate decisions" would need to be taken.
He insisted that the Government was not "seeking a mandate" in today's debate.
Mr Clegg said: "What we are seeking to do today in the House of Commons is to make the case for a simple limited response to what happened last Wednesday, nothing more, nothing less.
"Not a further set of initiatives, not a further set of steps. That is not what we're seeking, only this step. We would return to Parliament, we would consult again with our international allies and colleagues if further steps were necessary."
He added: "If we do nothing, then it is more likely, much more likely than not, that they (chemical weapons) will be used again and again and again. I don't want to inhabit a world, I don't want my children to inhabit a world, where chemical weapons become used on a much more widespread basis."