Britain and the US are facing calls to take some form of economic reprisal after China signalled that it was taking major steps to restrict civil liberties in Hong Kong by pushing through national security laws.

In an early sign of the British reaction, Lord Patten, the last British governor of Hong Kong and a passionate critic of Chinese regime, called the move a “comprehensive assault on the city’s autonomy, rule of law, and fundamental freedoms”.

He has recently written to the foreign secretary, Dominic Raab, seeking assurances that the UK will not allow the Sino-British joint declaration to be torn up. The declaration, signed in 1985, made clear that Hong Kong’s high degree of autonomy, rights and freedoms would remain unchanged for 50 years.

In the US, Senator Ben Sasse, a Nebraska Republican on the Senate Select Committee on Intelligence, condemned Beijing’s plans, accusing the Communist party of “trying to kill Hong Kong’s autonomy” and promising action if Beijing goes ahead.

“Beijing’s current proposal will effectively destroy the rule of law in Hong Kong and will give Chairman Xi’s thugs legal cover to treat freedom-loving protesters as terrorists,” Sasse said in a statement.

The chair of the House foreign affairs committee, Michael McCaul, warned: “Any law passed by the CCP that further stifles the freedom of the people of Hong Kong would only further erode the foundations of One Country, Two Systems, and will not be tolerated by the United States.”

Donald Trump has been piling new trade restrictions on China in recent weeks, partly in response to the Chinese handling of the coronavirus outbreak. But Republican Senators called for Trump in the specific case of Hong Kong to change the former UK colony’s preferential trading status. A US government determination of the special administrative region’s “autonomy” is a precondition for continued US preferential trade and investment terms distinct from those applied to China. Mike Pompeo, the US secretary of state, has already said it will be difficult to declare the region autonomous in view of recent arrests of protesters.

The US president seems determined to make the threat posed by the Chinese Communist party the defining issue in the campaign for November’s presidential election, so may need little encouragement to impose new economic tariffs. Steve Bannon, his onetime political aide, said Trump’s three election themes will be China, China, and China.

On Thursday, Trump told reporters at the White House that “nobody knows yet” the details of China’s plan. “If it happens, we’ll address that issue very strongly,” Trump said, without elaborating.

Given the long history of bipartisan scepticism towards China in Washington, Democrats are unlikely to oppose any measures. Indeed, pro-democracy legislators in Hong Kong such as Claudia Mo are already expecting Trump to respond to China’s actions. “China’s move might just be considered savage by outsiders, and there could be a huge political price to pay internationally,” she said. “Is it worth it?”

The UK government, by contrast, has been involved in a balancing act.

Tighter UK takeover laws are due to be published soon, but the UK Treasury does not believe decoupling between the west and China is in anyone’s interest. The UK is also the No 1 foreign direct investment site for China in Europe, even if trade between Germany and China dwarfs that of the UK.

Politically, the UK does not want to jettison China. Lord Hague, a former Conservative foreign secretary, said this month that there was no major challenge facing the world that could be solved without the cooperation of China. That applies especially to UN climate change talks, a process in which the UK has the diplomatic lead.

The UK intelligence agencies have also made the judgment that Chinese-owned Huawei firm could be at the heart of the UK’s new 5G network, without compromising national security, a decision that has led to heated phone calls between Trump and Boris Johnson.

But the pressure to change tack is growing. Tom Tugendhat, chair of the foreign affairs select committee, said this week UK relations with China had reached “a turning point”. He argued: “The UK has a special duty to respond since China is now directly and incrementally challenging the international rules-based system.”

The UK, he said, was dealing with a very different China from the China of 2010. “Up until a decade ago, we saw that increased wealth led to increased democratisation. Well, that is not so any more.”

Labour’s Asia spokesman, Stephen Kinnock, has also adopted a more active and assertive position.

But the perennial question is what in practical terms can the UK do to resist a takeover in Hong Kong, apart from express outrage. Extending the British National Overseas Passport offer to Hong Kong citizens is the normal demand.

In a muted letter to the foreign affairs select committee, Raab said: “If there is any attempt to include national security legislation before wider concerns with rights and freedoms in Hong Kong has been addressed satisfactorily, it could further unsettle the situation.”

The UK Foreign Office said: “We are following reports and monitoring the situation closely. We expect China to respect Hong Kong’s rights and freedoms and high degree of autonomy. As a party to the joint declaration, the UK is committed to upholding Hong Kong’s autonomy and respecting the ‘one country, two systems’ model.”