The UK will push for the European Union's arms embargo on Syria to be lifted to enable the potential supply of weapons to forces fighting against Bashar Assad's regime.
At a meeting in Brussels, William Hague will argue that there is a "compelling" case for easing the restrictions in order to strengthen the rebels' position.
The current embargo expires at the end of the month and Europe's foreign ministers will attempt to thrash out a replacement package of measures, with the Foreign Secretary and France arguing for changes to allow the supply of arms to "moderate" rebels.
But counterparts in other European nations are concerned about the danger of weapons falling into the wrong hands.
In a statement to Parliament last week Mr Hague said: "The case for further amendments to the arms embargo on Syria is compelling, in order to increase the pressure on the regime, and to give us the flexibility to respond to continued radicalisation and conflict.
"We have to be open to every way of strengthening moderates and saving lives rather than the current trajectory of extremism and murder.
"We have not sent arms to any side during the conflicts of the Arab Spring. No decision has been made to go down this route, and if we were to pursue this, it would be under the following conditions: in co-ordination with other nations, in carefully controlled circumstances, and in accordance with our obligations under national and international law."
He said the UK and France were "strongly of the view" that changes were essential to ensure that the Assad regime takes forthcoming negotiations in Geneva seriously.
"We must make clear that if the regime does not negotiate seriously at the Geneva conference, no option is off the table," he warned.
Reza Afshar, the head of the Foreign Office's Syria team, wrote on Twitter that the EU faced a "strategic decision" today: "Give hope to political track on #Syria by amending arms embargo; or kill off political track."
Earlier this month a Syrian rebel commander was filmed biting into a dead soldier’s lung. The clip, which was verified by Human Rights Watch (HRW), showed Khalid al-Hamad (who is also known as Abu Sakkar) hacking into the man’s body and removing organs before raising the lung to his lips.
He is heard saying: “I swear to God we will eat your hearts and your livers, you soldiers of Bashar the dog,” to offscreen cheering.
Rebel leader Abu Sakkar takes a bite out of the soldier's lung
The conflict has also claimed the life of a young London doctor who died in Syria after government forces shelled the temporary medical centre he was working in, according to a Syrian human rights group.
Dr Isa Abdur Rahman, 26. who graduated from Imperial College London, died after the Assad forces shelled a medical facility where he was working, according to Hand In Hand for Syria.