Foreign Secretary William Hague says the UK's commitment of an extra £5m in non-lethal equipment to Syrian opposition groups is "the right thing to do" and will "help save lives".
The help for unarmed opposition members will include medical supplies and radio and satellite equipment.
But it will not include weapons and is in addition to £27.5m humanitarian aid.
The BBC's James Robbins says the move is a significant shift in policy after frustration about Syria's opposition.
Aside from concerns about divisions within the opposition, there have also been complaints that it has failed to set out a clear programme for good government, our correspondent added.
Speaking at a press conference, Mr Hague said the funding would go to "unarmed opposition groups, human rights activists and civilians" and that Britain was speaking to people from the political arm of the Free Syrian Army, an armed rebel group which is opposed to President Bashar al-Assad.
Attempts to oust President Assad have led to 17 months of unrest, during which activists claim more than 20,000 people have died.
However, reports of casualties often cannot be independently verified.
The UK is also telling opposition forces they must observe human rights standards.
Syrian people need "urgent help" and "cannot wait indefinitely" for a peaceful resolution to the conflict, Mr Hague said, describing them as being at the "mercy of a regime that is hunting down its opponents".
The UN Security Council, which meets again at the end of this month, has a responsibility to "stop the bloodshed", he said.
Mr Hague said the money "will help people caught up in a terrible conflict. It will be delivered in co-ordination with other countries. It is the right thing to do, while not pausing for a second in our efforts to secure the united robust diplomatic action which this crisis demands."
The money will be spent on medical supplies such as trauma kits, surgical equipment, medicines and water purification and to assist local doctors with the means to gather forensic evidence for any potential trials in future.
The communications equipment includes mobile and satellite phones and radar equipment. The Foreign Office says it will help protect activists overcome the Syrian government's jamming and blocking techniques.
The foreign secretary added that body armour would be provided for civilians who were involved in the protection of others.
Mr Hague, who has also described the Assad regime as "doomed", said it remains British policy not to send arms to Syria.
BBC security correspondent Frank Gardner says the UK has been quite vocal at the UN but reluctant to get involved on the ground in Syria.
But the latest announcement is a realisation that the situation will continue to get worse if it does nothing, he added.
Former British foreign secretary, Sir Malcolm Rifkind, says the government should be doing more in Syria.
"The way in which you have to act is to actually take the very difficult decision of providing military support to the insurgents," he said.
"That way you may enhance the conflict in the short term, in the longer term you're more likely to save thousands of lives."
The UK had previously made £1.4m available in "non-lethal support to the political opposition" in Syria which included training and assistance to human rights groups.
The £27.5m in humanitarian aid is for food, medical care, water sanitation and shelter in Syria, as well as for Syrian refugees who have fled to neighbouring countries.
International Development Secretary Andrew Mitchell had announced this week that the amount of assistance being given to the refugees was being quadrupled.
It follows on from £18.4m in UK humanitarian assistance to Libya during the uprising against Colonel Gaddafi last year. This included supplying shelter for both refugees and those made homeless, supporting mine clearance and also helping the World Health Organization to provide medical care.