They were invited to work in Britain after World War II and given indefinite leave to remain, but many who failed to get their papers in order now risk being uprooted if they cannot prove their legal rig
More than 140 British lawmakers have called for action over the "inhumane" treatment of members of the so-called Windrush generation, named after the ship that brought over the first group of West Indian immigrants in 1948.
They were invited to work in Britain after World War II and given indefinite leave to remain, but many who failed to get their papers in order now risk being uprooted if they cannot prove their legal rights.
Immigration minister Caroline Nokes conceded that "we have made mistakes", revealing that in some cases people were asked to provide four pieces of documentary evidence for every year spent in Britain.
Leaders of the 53 Commonwealth countries are in London for their biennial heads of government meeting, and some member states have expressed alarm at the situation.
Guy Hewitt, the London-born High Commissioner to Barbados, told BBC radio that he felt "the country of my birth is saying to people of my region you are no longer welcome on my shores".
"Because they came from British colonies which were not independent they felt they were British subjects, they felt there was no need for them to recognise their status," he said.
"And now, 40, 50 years on they are being told by the Home Office that they are illegal immigrants. They are being shut out of the system, some of them detained, others have been deported."
A tightening crackdown on illegal immigration in recent years has led to more people being asked to prove they are legally entitled to be in Britain.
The prime minister's spokesman insisted that "no-one with the right to be here will be made to leave".
He confirmed that 12 Commonwealth leaders had requested a meeting and one was urgently being organised this week.
May "deeply values the contribution made by these and all Commonwealth citizens who have made a life in the UK", he said.
"If there have been problems which people have been put through, that clearly would be a matter of regret."
Opposition Labour lawmaker David Lammy, whose parents were from Guyana, organised the letter of cross-party MPs against what he said was a "grotesque, immoral and inhumane" situation.
The letter highlights how the uncertainty surrounding people's immigration status has affected their right to work, to rent homes, to receive pensions or even access healthcare.
"It is a stain on our nation's conscience," Lammy said, adding that the prime minister's decision to call a meeting was only a "small" concession.
He demanded guarantees for "all the Windrush children caught up in this crisis. Anything less than that is not good enough".
Commonwealth Secretary-General Patricia Scotland, who herself moved to Britain from Dominica when she was two years old, said the issue was one for individual countries to resolve with London.
She said it would be a "very challenging" week in general, adding: "Some of the internal issues have to remain just that, whatever your own personal views."