* Row over Caribbean immigrants who arrived as children
* Minister apologises for "appalling" treatment
* Lawmakers call on PM May to resolve problems
* Issue could sour Commonwealth summit in London (Recasts, adds interior minister comments)
By Andrew MacAskill
LONDON, April 16 (Reuters) - Interior minister Amber Rudd apologised on Monday to thousands of British residents who arrived from the Caribbean decades ago and are now being denied basic rights after being incorrectly identified as illegal immigrants.
Prime Minister Theresa May is under growing pressure to resolve the issue of the 'Windrush generation' of migrants who arrived in Britain more than 50 years ago and have become victims of a recent tightening of the immigration system.
More than 140 lawmakers have signed a letter calling on May to resolve an anomaly that means many people who immigrated as children between 1948 and 1971 are being denied health services or prevented from working. Intensifying the row, junior home office minister Caroline Nokes admitted on Monday that some people may have been deported in error.
Many have been told they need evidence including passports to continue working or getting health treatment despite living, working and paying tax in Britain for decades. Some arrived on their parents' documentation and never formally applied for British citizenship or a passport.
Rudd said on Monday a new unit would be established to help people establish their right to remain in Britain, and that if anyone needs to apply for new documents the fees will be waived.
"Frankly, some of the way they have been treated has been wrong, has been appalling, and I am sorry," she told parliament.
There is widespread anger that long-term British residents have fallen victim to rule changes in 2012 -- when May herself was interior minister -- aimed at stopping overstaying.
May's six-year tenure at the interior ministry was marked by a determination to reduce immigration numbers, something she has continued to emphasise as premier and in Brexit negotiations. In 2013, her ministry displayed billboards on vans telling illegal immigrants to "Go Home or Face Arrest."
The row has threatened to overshadow Britain's hosting of the annual Commonwealth Heads of Government meeting in London this week. Ministers have said they hope to tighten ties and increase trade with the Commonwealth, a network of 53 countries, mostly former British colonies, after Britain leaves the European Union next year.
The immigrants are named after the Empire Windrush, one of the first ships to bring Caribbean migrants to Britain in 1948, when Commonwealth citizens were invited to fill labour shortages and help rebuild the economy after World War Two.
Almost half a million people left their homes in the West Indies to live in Britain between 1948 and 1970, according to Britain's National Archives.
British media have reported cases such as a man who was denied treatment for cancer and a special needs teaching assistant who lost his job after being accused of being illegal immigrants despite living in Britain for more than 40 years.
"It's disgraceful that the rights of the Windrush Generation have been brought into question by this government and that some have been wrongfully deported," leader of the opposition Labour Party Jeremy Corbyn said.
The British government last week refused a request from the high commissioners of 12 Caribbean nations for a dedicated meeting on the subject at the Commonwealth meeting this week.
May only became aware of a request for a meeting on Monday morning, and will discuss the issue with counterparts from Caribbean nations this week, her spokesman said.
A Home Office official said the rejection had been because the subject of the meeting was not clear.
Under the 1971 Immigration Act, all Commonwealth citizens already living in the UK were given indefinite leave to remain.
But 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 Britain legally.
An online petition calling for an amnesty for those who arrived in Britain from Commonwealth countries in the Caribbean as children, and a lowering of the level of documentary proof required from people who have lived here since they were children, has now attracted more than 136,000 signatures.
(Additional reporting By Elizabeth Piper; Editing by Catherine Evans)
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