Seven former UK foreign secretaries have urged Boris Johnson to form a global alliance to co-ordinate the response to the China-Hong Kong crisis.
China is facing mounting criticism over a planned security law for Hong Kong which would make it a crime to undermine Beijing's authority.
Foreign Secretary Dominic Raab said the UK would not turn a blind eye.
Hong Kong was handed back to China from British control in 1997 but under a unique agreement.
The former British colony enjoys some freedoms not seen in mainland China - and these are set out in a mini-constitution called the Basic Law.
But there are fears the proposed law, which has sparked protests in Hong Kong, could compromise some of the freedoms guaranteed by the Basic Law.
In their letter to the prime minister, the cross-party group of former cabinet ministers says the UK government must be seen to lead the international response, as many countries take their cue from Britain over its former colony.
Jeremy Hunt, David Miliband, Jack Straw, William Hague, Malcolm Rifkind, David Owen and Margaret Beckett all expressed their concern at what they call China's "flagrant breach" of Sino-British agreements by imposing tough national security laws on Hong Kong.
They urged Mr Johnson to set up an "international contact group" of allies to coordinate any joint action, similar to that set up in 1994 to try to end the conflict in the former Yugoslavia.
A Downing Street spokesman insisted the government was already playing a leading role with international partners in urging China to think again.
Mr Raab said the new security legislation "very clearly violates" the autonomy that is guaranteed under Chinese law as well as that in the 1997 agreement.
He confirmed the UK will allow those who hold British National (Overseas) (BNO) passports to come to the UK and apply to study and work for an extendable 12-month period.
This will in turn "provide a path to citizenship", he told the BBC's Andrew Marr Show on Sunday.
BNO passports were granted to Hong Kong citizens born before the Chinese handover in 1997 and while they allow the holder some protection from the UK foreign service they do not give the right to live or work in Britain.
Mr Raab said up to three million people registered as a British national (overseas) in Hong Kong could be eligible for UK citizenship if China presses ahead with the law.
Meanwhile, the chairman of Commons foreign affairs committee, Tom Tugendhat, said the government must realise that China has a "very, very authoritarian system of government" and should rethink the partnership between the two.