More than two decades after Britain handed control over Hong Kong to Beijing, a British passport status that until recently appeared to have fallen out of time is suddenly gaining new relevance.

As the Hong Kong leadership — backed by Beijing — is increasingly cracking down on pro-democracy protests in the semiautonomous territory, some argue that the West’s most credible leverage over the Chinese leadership there hinges on the British National Overseas (BNO) passport category. Issued to those who were residents of Hong Kong before the transfer of power in 1997, the passports entitle holders to some but not all rights that British citizens can rely on.

Passport holders can, for instance, travel to Britain for up to six months without a visa or seek consular assistance abroad, but they are not entitled to stay in the U.K. indefinitely.

Calls on the U.K. government to grant BNO passport holders full British citizenship are gradually mounting, however, amid concerns that China may be breaking its promise to maintain Hong Kong’s semiautonomous status for at least three more decades.

Migration consultancies in Hong Kong have reported a surge in interest there in moving abroad in recent months, pulling Britain in particular further into the confrontation between pro-democracy protesters and Beijing.

In the fall, more than 100,000 people backed a petition to the U.K. government, demanding full citizenship for BNO passport holders. Hundreds of protesters rallied in front of the British consulate in Hong Kong in early September.

Craig Choy, a spokesman for the Equal Rights for British National Overseas campaign group, said in a phone interview that he had noticed a “positive momentum in the U.K.” to provide BNOs with “full citizenship and equality” in recent months.

That momentum has not translated into government action. In a response to this year’s petition, the U.K. Home Office wrote that “only British citizens and certain Commonwealth citizens have the right of abode in the United Kingdom” and that “there are no plans to amend the law.”

The Home Office cited a 2007 review, which found that giving BNO passport holders “full British citizenship would be a breach of the commitments made between China and the UK in the 1984 Joint Declaration on the future of Hong Kong, and that to secure Chinese agreement to vary the terms of that treaty would not be possible.”

But in recent days, the idea has gained new momentum, after the British House of Commons’ Foreign Affairs Select Committee embraced some of the proposed changes. In a report published Nov. 5, the committee argued that the current arrangement exposed BNO passport holders to “arrests by authorities” in Hong Kong and that, as a result, the U.K. government should extend “the right of abode to Hong Kong residents who are British National (Overseas) passport holders.”

The committee does not have the authority to force changes, and the government’s position appears unchanged.

Proponents of an adapted arrangement hope that granting passport holders more rights would create an escape option for Hong Kong residents vulnerable to Chinese government reprisals because of their political activism. As full citizens or passport holders with expanded rights — and unlike under the current arrangement — they would be able to travel to the U.K. and remain there for longer than six months.

But any serious efforts to implement changes would likely prove challenging. China could perceive unilateral British changes to the 1984 Joint Declaration that paved the way for the power transfer as a violation of the pact and as a provocation. In recent months, far more subtle British criticism of Beijing has drawn a fierce backlash.

It also remains unclear to whom exactly the proposed passport changes would apply. At the end of last year, Home Office documents suggested that almost 170,000 people held valid BNO passports — with the vast majority residing in Hong Kong. Other reports put the figure higher, at 248,000.

But the actual number of those who may qualify for full British citizenship if campaigners’ demands were to be accepted could be far higher.

Whereas almost 800,000 BNO passports were issued between April 1997 and December 2006, interest in renewing the documents appears to have faded in the ensuing years.

Some deemed the process too expensive and the passport itself of not enough value, which reportedly resulted in a 2005 campaign by the British Consulate in Hong Kong to shore up support for the passport under the slogan: “BNO: Yours for Life.”

“We realized we had to get a message out saying this is a nationality status which lasts a lifetime, it doesn’t just expire with the validity of your travel document,” a consulate spokeswoman told the South China Morning Post at the time.

Theoretically, whoever qualifies for a BNO passport is still eligible to renew the document. As of 2007, the British government assumed that the total number of Hong Kong residents eligible to apply or already in possession of a passport could be as high as 3.4 million.

That figure would be roughly equivalent to the total number of foreigners currently working in the United Kingdom, who became the target of a fierce immigration debate in the lead-up to the Brexit vote in 2016.

But campaigners doubt that the number is accurate, pointing out that it has remained unchanged since 1997, even though a significant share of former holders is likely no longer alive.

Campaigner Choy argued that, in any case, Hong Kong residents would be unlikely to move to Britain, even if they obtained full citizenship.

“Many people in Hong Kong love Hong Kong so much — we’d like to stay there,” he said.

With the territory’s leadership cracking down on protesters, Choy argued, those who have the greatest “confidence to stay” may be the ones who — theoretically — would have a secure option to leave.

Read more: