British Foreign Secretary Jeremy Hunt warned China on Tuesday of "serious consequences" if it breaches a bilateral agreement to preserve Hong Kong's freedoms, after an unprecedented night of anti-government protests.
"We expect that legally binding agreement to be honoured and if it isn't there will be serious consequences."
Asked to give details, Hunt said: "You have to allow me some latitude as foreign secretary to make a decision on that when the appropriate time comes. But I want to be absolutely clear that our values are not negotiable and we expect all countries to honour their legal agreements with the UK."
He added: "None of us support the violence that we saw on TV last night but we do understand the reason for people's anger."
Hunt was speaking during a campaign event in Northern Ireland as part of his bid to lead the ruling Conservative Party and become prime minister.
Junior foreign minister Alan Duncan said in London that the government rejected Beijing's assertion that the joint declaration was no longer valid.
"Our clear view is that the Sino-British Joint Declaration of 1984 obliges the Chinese government to uphold Hong Kong's high degree of autonomy and its rights and freedoms," he told parliament.
"We call on the Chinese government to do so in respect of the recent demonstrations.