A cross-party group of 140 MPs has written to Prime Minister Theresa May calling for an "immediate and effective" response to problems faced by long-term British residents from the "Windrush generation" over their immigration status.
The move follows concern that people are facing deportation and being denied access to healthcare due to UK paperwork issues and anomalies affecting some immigrants who arrived between the late 1940s and early 1970s.
International Development Secretary Penny Mordaunt insisted people had no need to be concerned, but acknowledged the Government needed to do more to allay fears.
The letter to the PM was co-ordinated by David Lammy, chairman of the Race and Community All Party Parliamentary Group, and has the backing of Jeremy Corbyn and Conservative MPs such as Sarah Wollaston.
Mr Lammy (pictured) said: "What is going on is grotesque, immoral and inhumane. It is a stain on our nation's conscience and the Prime Minister must act urgently to right this historic wrong.
"After World War II we invited the Windrush Generation over as citizens to help rebuild our country, and now their children are being treated like criminals.
"The government is essentially stripping people of the rights that our government itself granted decades ago.
"The government must immediately guarantee that anyone who comes forward to clarify their status should not face deportation or detention, because as things stand today there are thousands of people who are too worried about their future to come forward."
The letter to Mrs May calls for action over the immigration anomalies, stating: "All too often these routine bureaucratic errors bring about the separation of families and irreparable damage to lives in addition to undue stress, anxiety and suffering.
"The impact has been felt in the cases of individuals losing the right to work, to rent property, to receive pensions, to access their bank accounts or even to access vital healthcare - a particularly cruel twist of fate as so many of those affected have spent their lives in the service of our National Health Service."
Ms Mordaunt told BBC Radio 4's Today programme: "People who are in that situation, there is absolutely no question of their right to remain, and their right to gain access to services such as healthcare.
"What clearly needs to happen is we need to do a better job with the process that these individuals are having to go through.
"People should not be concerned about this - they have the right to stay and we should be reassuring them of that."
Barbados High Commissioner Guy Hewitt said it was "most unfortunate" Mrs May did not have space in her schedule to meet Caribbean leaders to discuss the issue at this week's Commonwealth conference in London.
He told the BBC: "Because they came from colonies which were not independent, they thought they were British subjects. They thought that there was no need for them to regularise their status.
"And 40, 50 years on are being told by the Home Office not that they are just anomalies, but they are illegal immigrants. They are being shut out of the system.
"Some have been detained, are still being detained. Others have been deported."
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