China's embassy in the United Kingdom has strongly protested about a recent joint letter regarding the situation in Hong Kong that was signed by seven former British foreign secretaries.

"China expresses grave concern about and strong opposition to such flagrant interference in Hong Kong affairs, which are China's internal affairs," an unnamed embassy spokesperson said on Tuesday.

In the letter, former UK foreign secretary Malcolm Rifkind together with six other former top UK diplomats expressed their "concerns" about the recent decision by China's top legislature, the National People's Congress, on national security legislation for the Hong Kong Special Administrative Region.

They urged Prime Minister Boris Johnson to raise this issue at the upcoming G7 meeting, and demanded formally instituting an International Contact Group to monitor the situation in Hong Kong, drawing inspiration from one that was formed in the 1990s to monitor developments in the Balkans.

In response, the embassy spokesperson said "Hong Kong affairs brook no external interference", and emphasized that the national security legislation was purely China's internal affair, in which no foreign country has the right to interfere.

The national security legislation for Hong Kong falls within the legislative power of the nation and the responsibility of the central government of China, the spokesperson said.

"It is wishful thinking that China would swallow the bitter fruit of external interference. China will respond firmly to any external interference in its internal affairs by any country or organization in any form," the spokesperson said.

Given the chaos in Hong Kong during the past year, it is right and responsible for the National People's Congress to "plug this loophole that compromises national security in Hong Kong through legislation in accordance with the constitution and the basic law", the spokesperson said.

The legislative move "has won full understanding and support from the international community", the spokesperson added, citing recent support voiced by countries and international organizations, including Russia, Serbia, Cambodia, and the African Union.

The embassy spokesperson also said the allegation that the NPC decision was a "breach of the Sino-British Joint Declaration" was "a false proposition".

"Any country who uses the joint declaration as an excuse to interfere in Hong Kong affairs is going against the principles of international law that there should be respect for sovereignty and territorial integrity and non-interference in other country's internal affairs," the spokesperson said.

In response to the issue of British Nationals Overseas passports, or BNOs, the spokesperson said the UK had clearly pledged in a memorandum of understanding exchanged with China that BNO passport holders who are Chinese citizens residing in Hong Kong shall not have the right of abode in the UK.

"If the UK is bent on changing this unilaterally, it will not only go against its own position and promise but also violate international law and the basic norms governing international relations," the spokesperson added.

The national security legislation for the Hong Kong SAR facilitates the better implementation of "One Country, Two Systems" and helps safeguard the rights and freedom of Hong Kong residents, the spokesperson said.

"It also answers the call of Hong Kong residents for better security safeguards so that they can enjoy and exercise their statutory rights and freedom, and their aspiration for a safer, better, and more prosperous Hong Kong," the spokesperson said. "We urge the relevant UK politicians to accept the fact that Hong Kong is now part of China, observe the principle of non-interference in other country's internal affairs, and stop meddling in Hong Kong affairs, which are China's internal affairs, in any form, by any means."