British Foreign Secretary William Hague said on Wednesday that non-lethal military aid, worth $20 million (15.4 million euros), was a "necessary, proportionate and lawful response" to a situation of "extreme" humanitarian suffering in Syria.
"The fact remains that diplomacy is taking far too long and the prospect of an immediate breakthrough is slim," he told parliament.
The announcement comes after the European Union authorised on Thursday last week the supply of non-lethal military equipment and training to the Syrian opposition.
Britain has already provided them with equipment such as power generators and communications devices worth £9.4 million ($14.2 million, 10.9 euros).
But Hague said his government was being forced to move towards "more active efforts" in a bid to end the violence.
"We will also now provide new types of non-lethal equipment for the protection of civilians, going beyond what we have given before," he said.
"It will certainly include, for instance, armoured four-wheel drive vehicles to help opposition figures move around more freely, as well as personal protection equipment including body armour."
Britain will also give the rebels equipment to test for the possible use of chemical weapons by the regime, he added.
US Secretary of State John Kerry said Wednesday that "a lot of countries" were already training the rebels as part of stepped-up efforts to end the brutal conflict, which the United Nations estimates has cost at least 70,000 lives.
One million Syrians have also fled their homeland since the revolt against Assad erupted two years ago, the UN said.
The United States last month pledged $60 million in funding for the political opposition as well as food and medical assistance, but not the weapons the rebels have asked for.
An EU arms embargo prohibits European countries from supplying weapons to the Syrian opposition, although Britain has been pushing to lift this.
Hague urged other countries to provide non-lethal help.
"We and the rest of the European Union will have to be ready to move further, and we should not rule out any option for saving lives," he said.
Hague is due to discuss the crisis with his Russian counterpart Sergei Lavrov next week, following a meeting with Lavrov's deputy, Mikhail Bogdanov, later on Wednesday.