“From the materials I have been able to review, it is my view that the UK government can extend full right of abode to BN(O) passport holders without breaching its side of the Sino-British Joint Declaration,” Goldsmith wrote in the letter dated February 14, a similar version of which was also sent to the Foreign and Commonwealth Office.

“I said in the 2008 citizenship review that it would be fair to grant greater rights to BN(O) passport holders. I continue to hold this position.”

A Home Office spokesman said: “We continue to believe that the best solution for Hong Kong, and the British National (Overseas) passport holders that live there, is full respect for the rights and freedoms guaranteed in the Sino-British Joint Declaration.”

According to a 2015 British government estimate, 3.4 million people hold BN(O)s, which Hongkongers could apply for before the 1997 return to Chinese sovereignty. Those born after the handover are not eligible.

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Last year, in rejecting British citizenship for Hongkongers with BN(O)s, the government cited the 2008 report: “Lord Goldsmith recognised that to automatically give BN(O)s 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.”

In his letter to the home secretary, Goldsmith – who was the British government’s chief legal adviser from 2001 to 2007 under Labour prime minister Tony Blair – said the assertion was “a mischaracterisation of what I said”.

“I want to make it clear: I never intended my report on citizenship to be a statement on any opinion by me that there would be a breach of the arrangements with China if the UK were to offer greater rights,” he said.

“I do not see why the UK government would be in breach of any obligation undertaken in the joint declaration were it to resolve to extend full right of abode to BN(O) passport holders while continuing to honour their side of the Sino-British Joint Declaration.”

Activists have compared the British government’s refusal to change BN(O) status to the Windrush scandal, where the Home Office threatened to deport thousands of people who arrived as children in Britain from the Caribbean decades ago.

“Goldsmith’s powerful letter has unmasked the UK and shown they have learnt nothing from the Windrush scandal,” said Luke de Pulford, fellow at the London-based Hong Kong Watch group and the campaigner who first contacted the former attorney general for clarification.

“It is time for the home secretary to dismiss baseless legal objections from the Foreign Office and to give British nationals in Hong Kong what they need and deserve.”

Goldsmith also suggested that the British government had been relying on a clause that bound it only up to the 1997 handover to justify their denial of BN(O) citizenship today.

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At question is a memorandum on the right of abode for BDTCs – British Dependent Territories Citizens – that the British negotiation team presented to their Chinese counterparts in the 1980s.

It was under this unilateral memorandum, which Goldsmith said the Chinese side “neither accepted nor agreed to”, that BDTCs were converted to BN(O)s without the right of abode.

“The exchanged memoranda cannot be interpreted as imposing a permanent restriction on how the United Kingdom should treat the status of BN(O)s,” Goldsmith said.

The intention for the BDTC conversion to BN(O) “has already been discharged”, he said, adding: “The memorandum has no relevance to the UK’s present consideration of what rights should be enjoyed by holders of the BN(O) passports in the future. That is a matter solely for the UK.”

Prime Minister Boris Johnson’s government was understood to have revisited citizenship issues for Hongkongers last year, but no public announcement has been made.