Syria peace talks must take place "in good faith", Foreign Secretary Philip Hammond said as he welcomed the main opposition group's decision to attend.
The Higher Negotiating Committee (HNC) said it dropped a boycott after receiving assurances that humanitarian issues would be addressed.
But it insisted that it would not enter negotiations in Geneva until conditions were met - including an end to the bombardment of civilians by Russian and Syrian government forces, a lifting of blockades in rebel-held areas and the release of detainees.
The talks are the first stage of a United Nations-brokered process aimed at ending the civil war which has claimed at least 250,000 lives, allowed the self-styled Islamic State to take large swathes of territory and left neighbouring countries and Europe struggling to deal with huge numbers of refugees.
Delegations are not expected to come face to face at this stage, with UN special envoy Staffan de Mistura acting as go-between in a hoped-for dialogue.
Mr Hammond, who was among Western figures who urged HNC chairman Riyad Hijab to participate, said: "I welcome the difficult decision that the Syrian High Negotiating Committee has made to attend peace talks hosted by the United Nations special envoy Staffan de Mistura in Geneva.
"Intra-Syrian peace negotiations must deliver a political transition away from Assad and end the suffering of the Syrian people.
"The UK supports this process and calls for complete and unfettered humanitarian access across Syria, and an end to all violations of international humanitarian law, as set out in UN Security Council Resolution 2254.
"The UK believes in a Syrian-led political settlement to the conflict and expects both sides to participate in good faith to finally provide the peace the Syrian people deserve."
It came after the Netherlands agreed to join air strikes on IS targets in Syria.
As the Government prepared to host an international summit in London next week aimed at securing funding pledges for Syria, International Development Secretary Justine Greening said it would continue to match donations to Unicef UK's Syria Winter Appeal for an additional month.
The support was due to finish at the end of January.
Around £3 million has been donated so far by the public to the latest appeal.
Ms Greening said: "It's been such an amazing response from the British people that we've decided to extend our match funding for another month.
"The whole point of this conference is to make sure the rest of the world steps up and plays its part in responding to the crisis. Together, we will send a clear message to the children of Syria that the world has not forgotten them."
Actor Ewan McGregor, a Unicef UK Ambassador, welcomed the move.
He said: "As the brutal war enters its sixth year, the violence has not ceased and the suffering experienced by children and their families has deepened.
"A whole generation of children are being deprived of education and traumatised by the horrors of war. Increasingly they see their future shaped only by violence and bloodshed.
"As the spotlight shines on London next week, when world leaders come together to pledge money for this humanitarian crisis, the public can support the children of Syria by donating."