Cardinal Murphy-O'Connor said migrants can be 'easily exploited'
Long-term immigrants should be given British citizenship, according to the leader of the Roman Catholic Church in England and Wales.
Cardinal Cormac Murphy-O'Connor spoke out after Mayor of London Boris Johnson mooted an amnesty for immigrants.
The cardinal told BBC Radio 4's Sunday programme many newcomers to the country were "vulnerable" and needed support.
However, Sir Andrew Green, chairman of Migrationwatch UK, said talk of an amnesty was "frankly irresponsible".
"The experience of other countries like Italy and Spain is that amnesties simply produce yet more applicants every time," he said.
Migrationwatch estimates an amnesty would cost the taxpayer at least £500 million in extra benefit payments but supporters of such a move say it would boost the economy.
But Cardinal Murphy-O'Connor said there was a responsibility for the country to welcome immigrants to "appreciate the gifts that they bring and also make sure that in some way they are supported".
He continued: "A lot of the people who come are quite vulnerable and can easily be threatened and exploited.
"But I think also there is a point here about some migrants who come here and are here for years and they are undocumented.
"After a certain time a way should be given for them to receive citizenship here and so get the benefits of that."
Last week, Mayor of London Boris Johnson announced advisers were studying the potential benefits of an amnesty for the UK's estimated 700,000 illegal immigrants.
He said allowing long-term illegal immigrants to earn the right to stay in Britain would see "hugely increased" tax revenues, setting him against Conservative leader David Cameron.
However, immigration minister Phil Woolas has said any amnesty could lead to more people being exploited by traffickers.
He has previously suggested the UK population could be limited to 70 million.