BEIJING, Jan. 29 (CNA) - China said on Friday (Jan 29) that it would not recognise the British National Overseas (BNO) passport as a valid travel document or for identification starting from Jan 31.

The announcement comes as Britain prepares to open its doors to millions more residents of the former colony, following a security crackdown by Beijing.

British Prime Minister Boris Johnson's government has promised to provide long-term sanctuary for Hong Kong residents who want to leave the territory.

Holders of BNO status - a legacy of UK rule over Hong Kong up to 1997 - will from Sunday be able to apply to live and work in Britain for up to five years, and eventually seek citizenship.

BNO passport holders previously had only limited rights to visit the UK for up to six months, and no right to work or settle.

Beijing was swift to hit back at the British change on Friday.

"From Jan 31, China will no longer recognise the so-called BNO passport as a travel document and ID document, and reserves the right to take further actions," foreign ministry spokesman Zhao Lijian told reporters.

London says it is acting in response to a National Security Law imposed by China last year which has devastated Hong Kong's protest movement and rolled back freedoms meant to last 50 years under the 1997 handover accord.

Zhao said an "indignant" China believed Britain had marched well beyond the scope of the agreement, therefore nullifying it.

"The UK is trying to turn large numbers of Hong Kong residents into second-class UK citizens ... and has already completely changed the nature of the BNO," Zhao added.

It is unclear what, in practical terms, China's declaration means.

But it makes good on Beijing's threat to respond to Britain's extended visa offer with some sort of punitive reciprocal measures.

The threat of further action suggests Beijing may be preparing more restrictions for BNO holders down the line.

Chinese officials already warned last year that they might consider ending recognition of BNO passports. At the time they said it would mean BNO holders being unable to travel to the Chinese mainland.

However, it is unclear whether Chinese authorities would know who holds the document.

Hong Kongers use their own Hong Kong passport or ID card to leave the city. To enter mainland China, they need to use their Hong Kong passport. The only time they might use a BNO is on arrival into Britain or another country that recognises the document.