The Foreign Secretary welcomed the development, which he said showed a commitment to a “unified and pluralistic” Syria.
The formation of a unified opposition delegation is a major plank of the strategy for negotiations with president Bashar Assad’s regime on a political solution to the bloody civil war.
A peace plan agreed to last month by world powers meeting in Vienna set a January 1 deadline for the start of talks between Assad’s government and opposition groups.
The disparate groups agreed on a common framework following two days of talks in Riyadh.
Mr Hammond said: “I congratulate Syrian opposition leaders and their hosts, Saudi Arabia, on bringing together a broad and inclusive group of oppositionists to affirm their vision for Syria, develop a clear approach to negotiations with the regime, and appoint a negotiating committee which will now prepare the way ahead.
“I welcome the Riyadh Conference’s commitment to a unified and pluralistic Syria and its rejection of terrorism in all its forms.
“The UK will continue to play our full part in the International Syria Support Group (ISSG) established to ensure a Syrian-led and Syrian-owned political transition based on the Geneva communique of 2012, and to an ambitious but clear timetable.
“This conference was an important step ahead of the next ISSG meeting next week.”