David Cameron will urge European Union leaders to agree to arm the rebels trying to topple President Bashar al-Assad in Syria.
The EU moved a step closer to lifting its embargo on providing military weapons to the opposition forces yesterday.
The Prime Minister persuaded his fellow leaders to declare the Assad regime "illegitimate" and to ask EU foreign ministers "to work on all options to support and help the opposition and to enable greater support for the protection of civilians".
Mr Cameron warned an EU summit in Brussels that "a desperate crisis" is taking place "on our watch", adding: "People will ask in future years what did you do to help deal with this situation, to help bring about a transition to help get rid of President Assad?"
The Prime Minister conceded that EU governments are divided over arming the rebels. It is understood that Germany, Italy, Cyprus and Bulgaria are among the nations opposing the move. One EU diplomat accused Mr Cameron of "playing catch-up" after France officially recognised the Syrian opposition before Britain.
The Independent revealed this week that General Sir David Richards, the Chief of the Defence Staff, had chaired talks with other countries about military options in Syria. Mr Cameron said yesterday: "Syria is different to Libya. There are extra complications and difficulties. But instead of saying what we can't do, we should be asking what we can do."
Eurosceptic Conservative MPs expressed concern after Mr Cameron signed up to greater co-operation on defence at EU level. His critics warned that the move would undermine Nato.