Thousands of people who arrived in the UK as children in the first wave of Commonwealth immigration face being threatened with deportation.
They have lived and worked in the UK for decades but many are now being told they are here illegally.
A new petition on the government's website calling on the Home Office to grant them an amnesty has attracted more than 23,000 signatures.
The Home Office said it would handle applications to stay "sensitively".
The problem arises from the fact that under the 1971 Immigration Act, all Commonwealth citizens already living in the UK were given indefinite leave to remain - but the right to free movement between Commonwealth nations was ended from that date onwards.
However, the Home Office did not keep a record of those granted leave to remain or issue any paperwork confirming it, meaning it is difficult for the individuals to now prove they are in the UK legally.
The Migration Observatory at Oxford University estimates there are 500,000 people resident in the UK who were born in a Commonwealth country and arrived before 1971.
"This is a slap in the face for the Windrush generation," said Patrick Vernon, who started the petition, a reference to the Empire Windrush, which brought workers from the West Indies to Britain in 1948.
People who had "worked hard, paid their taxes, raised children and see Britain as their home" were being "threatened and harassed" by the Home Office in what he described as "an historic injustice".
It was particularly ironic, he added, because 2018 was the 70th anniversary of the Empire Windrush's arrival at Tilbury docks, in Essex.
He said his aim was to gain the 100,000 signatures needed to trigger a debate in Parliament and force the government to stop deportations, establish an amnesty and pay compensation to those who had suffered "loss and hurt".
Labour MP David Lammy, who is backing the campaign, tweeted: "We invited people as citizens, Home Office treating them like criminals."
** I am holding an event in Parliament on this on 1 May with those affected. If you have been affected, know people who have been affected or campaign on these issues please email me. We invited people as citizens, Home Office treating them like criminals. https://t.co/x8P3V2z2ue— David Lammy (@DavidLammy) April 10, 2018
People born in Jamaica and other Caribbean countries are thought to be more affected than those from other Commonwealth nations, as they were more likely to arrive on their parent's passports without their own ID documents.
Many have never applied for a passport in their own name or had their immigration status formalised, as they regarded themselves as British.
The Guardian newspaper has highlighted a number of cases of such people being threatened with deportation.
Earlier this year, the Labour peer Lord Faulkner raised the case of Paulette Wilson, a former cook at the House of Commons, who had come to Britain from Jamaica in 1968.
"(She) had never applied for a passport because she assumed she would not need one if she did not intend to travel abroad," he told the House of Lords.
"One day, she got a letter from the Home Office telling her to register each month at the Solihull immigration centre.
"While she was there on a visit, officials declared that she was an illegal immigrant, had her carted off to the appalling Yarl's Wood immigration removal complex and told her that she would be deported - presumably back to Jamaica, which she had not visited since she left as a child almost 50 years before," he added.
She was saved from deportation by the intervention of her MP, Emma Reynolds, and the Refugee and Migrant Centre in Wolverhampton.
She has now been given leave to remain, said Lord Faulkner, "although she has lost benefits for the past two years, as well as her flat, and has to rely on financial support from her daughter".
He said there were "many others" like Mrs Wilson - and he blamed measures introduced by then Home Secretary Theresa May in 2013 to make the UK a "hostile environment" for illegal immigrants, restricting access to jobs, driving licences, benefits, health care and accommodation.
He said this had "clearly led to overzealous interventions by officials".
A Home Office spokesperson said: "We value the contribution made by former Commonwealth citizens who have made a life in the UK.
"We want to assure individuals who have resided in the UK for an extended period but feel they may not have the correct documentation confirming their status that there are existing solutions available.
"They should take legal advice and submit the appropriate application with correct evidence so we can progress the case.
"We have no intention of making people leave who have the right to remain here," the Home Office added.
The Joint Council for the Welfare of Immigrants said there were cases of Australian, Canadian and South African, Indian and Pakistan-born citizens facing the same problem.
But it was "not as likely you will be asked to demonstrate your immigration status" by landlords or officials if you were "white and of European origin", JCWI chief executive, Satbir Singh, told BBC News.
The JCWI is calling for Commonwealth citizens who arrived before 1971 to be given the same status as EU citizens living in the UK, who will face a much lower burden of proof of residence than the Commonwealth citizens after Brexit.
The final details have yet to be confirmed but EU citizens may be able to produce things like school registration paperwork or library cards to prove residence, something that would not be accepted from Commonwealth citizens, the campaign group said.