China has betrayed the people of Hong Kong so the West should stop kowtowing to Beijing for an illusory great pot of gold, said Chris Patten, the last governor of the former British colony.

Beijing is set to impose new national security legislation on Hong Kong after a sustained campaign of pro-democracy protests last year in the city, which enjoys many freedoms not allowed on mainland China.

"The Hong Kong people have been betrayed by China," Patten was quoted as saying by The Times newspaper. Britain has a "moral, economic and legal" duty to stand up for Hong Kong, he said.

Patten watched as the British flag was lowered over Hong Kong when the colony was handed back to China in 1997 after more than 150 years of British rule.

Hong Kong's autonomy was guaranteed under the "one country, two systems" agreement principle enshrined in the 1984 Sino-British Joint Declaration signed by then Chinese Premier Zhao Ziyang and British Prime Minister Margaret Thatcher.

But China's plans to impose national security laws on Hong Kong risk destroying the Declaration, Patten said. The United States has branded the laws a "death knell" for the city's autonomy.

"What we are seeing is a new Chinese dictatorship," Patten said. "The British government should make it clear that what we are seeing is a complete destruction of the Joint Declaration."

Hong Kong leader Carrie Lam said her government will "fully cooperate" with the Chinese parliament to safeguard national security, which she said would not affect rights, freedoms or judicial independence.

Patten said the West should stop chasing the illusory promise of Chinese gold.

"We should stop being fooled that somehow at the end of the all the kowtowing there’s this great pot of gold waiting for us. It’s always been an illusion," Patten said.

"We keep on kidding ourselves that unless we do everything that China wants we will somehow miss out on great trading opportunities. It’s drivel."

The British government did not immediately comment on Saturday.

Prime Minister Boris Johnson's spokesman said on Friday the government was monitoring the situation and as a party to the Joint Declaration the UK was committed to the upholding Hong Kong's autonomy and respecting the one country, two systems model.