Thousands of the poorest and most vulnerable in society are being denied access to justice as a result of civil legal aid cuts, Amnesty International said.
Changes to legal aid rules in England and Wales were creating a "two-tier justice system" open only to those who have the money to pay, a report by the human rights organisation concluded.
When the Legal Aid, Sentencing and Punishment of Offenders (LASPO) Act came into force in 2013, there was a "staggering" fall-off in the number of cases where aid was granted - from 925,000 to 497,000, the report said.
Amnesty called on ministers to launch an immediate review of the system which it said had clearly been shown to be failing.
Researcher Alice Wyss said: "Cuts to legal aid imposed by this Government have decimated access to justice and left thousands of the most vulnerable without essential legal advice and support.
"We are in danger of creating a two-tier civil justice system, open to those who can afford it, but increasingly closed to the poorest and most in need of its protection.
"If Theresa May is really determined to deliver a country that works for all then there needs to be a justice system for everyone, not just those who can afford it."
Children and vulnerable young people were among those hardest hit by the changes, the report warned.
"Children and vulnerable young people cannot be expected to navigate complex legal processes alone, yet that is precisely what LASPO allows for.
"The impact has also been felt in cases where the best interests of the child are very much at stake," the report said.
"In human rights terms, the cuts to legal aid constitute a retrogressive measure.
"There is no dispute that fewer people can now access free legal help and representation in a wide range of cases."