Britain on Thursday summoned China's top diplomat in London after accusing Beijing of breaking its international treaty obligations by ousting four pro-democracy lawmakers from Hong Kong's legislature on security grounds.

Prime Minister Boris Johnson's spokesman said Chinese ambassador Liu Xiaoming had been called into the Foreign Office to hear London's "deep concerns at these latest actions".

"The UK will continue to call out these violations of Hong Kong's rights and freedoms and hold China to the obligations it freely assumed under international law," he added.

Hours earlier Foreign Secretary Dominic Raab told British lawmakers the new rules to disqualify elected assembly members was "a clear breach of the legally binding Sino-British Joint Declaration" agreed in 1984.

'Broken promises'

"China has once again broken its promises and undermined Hong Kong's high degree of autonomy," he said in a statement.

Britain is the former colonial power in Hong Kong and has increasingly locked horns with China since Beijing imposed a sweeping national security law last year.

Liu responded with a robust defence of Beijing's position.

"This decision of #HongKongSAR is justified & reasonable. Nowhere in the world are holders of public office, legislators included, ever allowed to breach their oath & betray their country," the envoy tweeted.

Fifteen legislators quit in protest at the ouster of their colleagues, confirming fears among critics that democracy is being eradicated in Hong Kong.

On Wednesday, Raab said China was seeking to "harass, stifle and disqualify democratic opposition," denting its international reputation.

London has angered Beijing by offering Hong Kongers holding British National Overseas passports a route to UK citizenship by relaxing entry and residency requirements.

It has also suspended its extradition treaty with Hong Kong, one of a number of countries to do so.

Junior Foreign Office minister Nigel Adams told parliament China had breached its treaty obligations three times since 1997, and twice in six months.

The treaty was signed long before Britain handed back Hong Kong to China in 1997 and was designed to allay fears about its future under Beijing's rule.

It guarantees the financial hub special status including a high degree of autonomy to manage its own affairs and the right to freedom of speech.

Adams told lawmakers the latest move was part of a "pattern of measures to pressure and stifle all voices critical of China's policies".

Britain has faced criticism in recent months for saying it planned to breach its own international agreement with the European Union over Brexit in a "very specific and limited way".

But Johnson's spokesman said other countries should recognise the "exceptional circumstances" the UK is in regarding proposed breaches of its EU divorce deal.

"There's no way that you can extrapolate from our approach to these very specific parts of the withdrawal treaty to the UK's international behaviour more generally," he said.

"As a country we stand for international law and the rules-based international system and we always will."

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