Four years after Britain voted to “take back control” of its borders in a Brexit referendum dominated by immigration, Boris Johnson's open-door offer to almost 3m Hong Kong citizens was greeted without a murmur of opposition at Westminster.

The UK prime minister's promise of a “route” out of the former British colony for millions of residents in the wake of the Chinese security crackdown won cross-party support in parliament, including from those who opposed uncontrolled EU migration.

“The left and the right of the party is in lockstep on this issue,” said Alan Mak, a Conservative MP of Chinese descent. “This is a very positive step. It's the right decision.”

In the Brexit referendum, Mr Johnson led a Leave campaign which raised the remote prospect of Turkish accession to the EU as an indication that Britain could soon become a destination for 80m Turks.

Mr Johnson, who has Turkish ancestors and is said by allies to be “pro-immigration”, is introducing a “points-based” system for migrants to Britain after Brexit and will end free movement for EU citizens.

But under his new proposal up to 3m Hong Kongers will be exempt: they will be able to work in Britain without having to meet any income test and eventually apply for citizenship. There will be no cap on numbers.

Mr Mak believes not many Hong Kongers will make the journey to Britain, but they will be welcomed if they do. “Ultimately this is a human rights issue, rather than an economic migration issue,” he said. In the House of Commons, no MP criticised Mr Johnson's proposal.

The policy is all the more striking when considering how John Major's Conservative government — with the support of Labour leader Tony Blair — refused the same rights to Hong Kongers before the 1997 handover to China.

Chris Patten, the last UK governor of Hong Kong and former Tory minister, and Paddy Ashdown, the Liberal Democrat leader at the time, pleaded with Mr Major to accept responsibility for residents in the colony in the face of an uncertain future.

“I don't think more than 3m Hong Kong citizens are suddenly going to arrive at Heathrow,” Lord Patten said in 1995. “To be blunt if they did they certainly wouldn't be living on the welfare state.”

In the event some Hong Kongers headed for Vancouver and San Francisco where they made new lives and — in many cases — fortunes. But at the time many Conservative MPs were not keen to see them settle in the UK.

Michael Heseltine, a former member of the Thatcher and Major governments, said Britain had not granted settlement rights to Hong Kongers because it would have been seen as “a major insult to China” and would have undermined the peaceful handover of the colony.

Norman Tebbit, a former Tory chairman, said in 1995 that Lord Patten had been happy enough to restrict Hong Kong immigration to Britain while he was an MP representing the spa city of Bath, but he was now representing the “people of Kowloon”.

The architects of Mr Johnson's proposal to offer new rights to Hong Kongers to move to Britain were Priti Patel, home secretary, and Dominic Raab, foreign secretary, who both come from immigrant families.

Mr Raab is the son of a Czech-born Jewish father who came to Britain as a refugee just before the second world war. Ms Patel's parents, Ugandan Asians, moved to Britain in the 1960s shortly before their community was expelled by Idi Amin.

“They are both here because of the generosity of the UK”, said one UK government official. “It's definitely personal for both of them and something they feel is the right thing to do.”

Some 27,000 Asian Ugandans took refuge in Britain in the 1970s after being evicted by Idi Amin. Like Hong Kongers, those arriving from Uganda had a reputation for hard work and enterprise.

UK officials said polling had shown the majority of Britons supported Mr Johnson's proposal for Hong Kongers and there was a belief public opinion will continue to back the government.

Officials also said there were no estimates of how many Hong Kongers will come to Britain or any plans to house the new arrivals in the event of mass migration. Much will depend on the way events play out in the coming months and years as China tightens its grip on Hong Kong.

Britain's relations with Beijing are now set on a completely different trajectory to the “golden era” envisaged by former Tory prime minister David Cameron, who opened up the British economy — including nuclear power and telecoms — to Chinese companies.

The decision to allow Huawei, the Chinese telecoms equipment maker, to supply kit for Britain's 5G mobile phone networks is now under review as Mr Johnson is egged on by US president Donald Trump to adopt a much tougher stance towards Beijing.