BEIJING/LONDON (Reuters) - China said on Friday the joint declaration with Britain over Hong Kong, which laid the blueprint over how the city would be ruled after its return to China in 1997, was a historical document that no longer had any practical significance.

In response, Britain said the declaration remained in force and was a legally valid treaty to which it was committed to upholding.

The stark announcement from the Chinese Foreign Ministry, that is sure to raise questions over Beijing’s commitment to Hong Kong’s core freedoms, came the same day Chinese President Xi Jinping said in Hong Kong the “one country, two systems” formula was recognized “by the whole world”.

It was not immediately clear if Foreign Ministry spokesman Lu Kang was attacking just the idea of continued British involvement in Hong Kong, which marks the 20th anniversary of Chinese rule on Saturday, or the principles in the document.

The Sino-British Joint Declaration, signed in 1984 by then British Prime Minister Margaret Thatcher and Chinese Premier Zhao Ziyang, laid out how Britain would end its century-and-a-half long rule over Hong Kong. It also guarantees the city’s rights and freedoms under the “two systems” formula.

Under Hong Kong’s mini-constitution, the Basic Law, Hong Kong was guaranteed its freedoms for “at least 50 years” after 1997.

Lu told reporters during a regular briefing on Friday that the document no longer binds China.

“Now Hong Kong has returned to the motherland’s embrace for 20 years, the Sino-British Joint Declaration, as a historical document, no longer has any practical significance, and it is not at all binding for the central government’s management over Hong Kong. The UK has no sovereignty, no power to rule and no power to supervise Hong Kong after the handover,” Lu said.

Britain said it had a legal responsibility to ensure China abided by its obligations under the declaration.

“The Sino-British Joint Declaration remains as valid today as it did when it was signed over 30 years ago,” a British Foreign Office spokeswoman said.

“It is a legally binding treaty, registered with the U.N. and continues to be in force. As a co-signatory, the UK government is committed to monitoring its implementation closely.”

On Thursday, British Foreign Secretary Boris Johnson said Britain hoped that Hong Kong would make more progress toward democracy.

“Britain’s commitment to Hong Kong – enshrined in the Joint Declaration with China – is just as strong today as it was 20 years ago,” Johnson said. “I’ve no doubt that Hong Kong’s future success will depend on the rights and freedoms protected by that treaty.”