British Prime Minister Boris Johnson is facing increasing pressure to give full citizenship to Hongkongers holding British National (Overseas) passports, or BN(O)s, as he runs for re-election.

A former Conservative Party chairman has joined the call for Johnson, who now leads the party, to correct the “historic error” of not giving BN(O) holders full citizenship, while the Liberal Democrats became the first major party to support in their election manifesto a “right of abode” for these Hongkongers.

BN(O) passports are travel documents that the British government granted only to Hongkongers born before the 1997 handover. As they do not entail the right to live and work in Britain, there have been calls to upgrade BN(O)s to full citizenship status amid the protests and increasingly violent clashes with police in Hong Kong since June.

The British government has been studying what possible changes could be made but the parliamentary election has added difficulty to an already complex and sensitive matter, which may weigh directly on Britain-China relations, several sources say.

The debate brought another layer of diplomatic pressure facing Hong Kong’s embattled government, after the US Senate this week passed a bill supporting Hong Kong democracy.

That bill, also passed by the US House of Representatives, has been sent to the White House and awaits President Donald Trump’s signature to become law.

The letter sent to Johnson on Thursday was signed by eight parliamentarians, including a number of prominent political figures in the House of Lords, among them Norman Tebbit, a former chairman of the prime minister’s own Conservative Party.

“The BN(O) passports were a historic error. Unlike in other colonies, the UK unilaterally revoked the residency rights of all Hongkongers without consulting them. This included people who fought in the British army and who served in the police force,” the letter stated.

“We hope you act boldly to take this important decision before it is too late.”

Others from Parliament’s upper house who signed the letter included Archie Hamilton and Ian McColl, former parliamentary private secretaries to the former prime ministers Margaret Thatcher and John Major, as well as Catherine Meyer, a former British ambassador to the US.

“By increasing the rights of BN(O) passport holders, we can not only correct this historic error, but also we can provide the support that these British nationals in Hong Kong vitally need at this sensitive time,” the letter added, noting that tens of thousands of people marched outside the British consulate in Hong Kong calling for the change.

According to a 2015 British government estimate, there are 3.4 million holders of BN(O)s, which Hongkongers were eligible to apply for before the 1997 handover. Those born after the handover are not eligible.

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Johnson has so far shied away from the topic of Hong Kong among campaign issues, which have been dominated by Brexit, the national health service and taxation. The election is to be held on December 12.

In contrast, the Liberal Democrats included in their election manifesto a statement to “honour our legal and moral duty to the people of Hong Kong by reopening the British National Overseas Passport offer, extending the scheme to provide the right to abode to all holders”.

The Liberal Democrats were the main winners in the European elections in London this year, fuelled by Londoners’ preference for the party’s anti-Brexit stance.

But in terms of domestic elections, the party is out of a political mainstream defined by the Conservative and Labour parties.

Explaining the Liberal Democrats’ support for Hongkongers, Chuka Umanna, the party’s foreign affairs spokesman, wrote in an article for the Independent on Wednesday: “The people of Hong Kong are not making unreasonable demands. They are simply calling for the promises that were made to them to be upheld – these promises enshrined in the Sino-British declaration.

“The Conservatives have let down the people of Hong Kong in their hour of need.”

A Liberal Democrat government would reopen the offer of BN(O) passports to Hong Kong citizens, he said, noting that around 169,000 Hong Kong residents hold active passports under this scheme.

“We would also extend the BN(O) passport so it provides the right of abode. This would give Hong Kong citizens the right to live or work indefinitely in the UK without any immigration restrictions or the need for a visa,” Umanna said.

There was no indication whether Thursday’s letter would have any immediate impact.

In September Dominic Raab, the foreign secretary, ruled out changing the status of the passports “for the moment” for fear of upsetting the balance reached in the Sino-British Joint Declaration in 1984, which first laid out terms for the 1997 handover.

Britain, he told parliament, was seeking “not to change the status of any one part of that package, but rather to press all sides, including China, to respect the delicate balance reflected in that package”.

“That is why, for the moment, we will not change or alter the status of the BN(O)s, but we will make sure that, in terms of their rights and entitlements they are entitled to expect within that status, they have our full support,” Raab said at the time.