Cuts in legal aid provision would lead to hundreds of thousands being denied the "help and justice" they needed, a volunteer organisation has claimed.
Citizens Advice issued a warning after the UK's most senior judge had raised concerns about legal aid cuts.
But ministers defended planned budget cuts and said legal aid would still be provided for those who most needed it.
"Legal aid cuts mean over half a million people in need of in-depth debt, benefits, family, housing and employment advice will be denied the help and justice they need," said Citizens Advice chief executive Gillian Guy.
"There will be a justice gap between those who can afford to pay for legal advice and those who can't. The poorest will be unable to challenge bad decisions about benefits, unfair dismissal at work and struggle to manage any debts they might have."
Ms Guy added: "Legal aid benefits advice helps the most vulnerable people secure the income they are entitled to by law, which puts food on their table and allows them to heat their homes.
"By nipping problems in the bud before they escalate and spiral out of control, Citizen Advice Bureau volunteers help to keep people out of court and facing costly legal proceedings.
"Legal aid cuts threaten this, introducing additional pressures for staff, new costs for clients and new costs to the taxpayer.
"The Government's expected £350 million savings will be a drop in the ocean compared to the cost these cuts will have on people's lives."
"My worry is the removal of legal aid for people to get advice about law and get representation in court will start to undermine the rule of law because people will feel like the government isn't giving them access to justice in all sorts of cases," said Lord Neuberger, in a BBC radio interview.
"And that will either lead to frustration and lack of confidence in the system, or it will lead to people taking the law into their own hands."
But ministers said the legal aid system cost about £2 billion a year and was one of the most expensive in the world.
A spokeswoman for the Ministry of Justice (MoJ) said the system was funded by taxpayers and resources were limited.
"At around £2 billion a year we have one of the most expensive legal aid systems in the world. We firmly believe it is an essential part of the justice system, but can never lose sight of the fact it is paid for by taxpayers and resources are not limitless," said the spokeswoman in a statement.
"Legal aid will continue to be provided to those who most need it, such as where domestic violence is involved, where people's life or liberty is at stake or the loss of their home. But in cases like divorce courts should be a last resort, not first. Evidence shows that mediation can often be more successful and less expensive for all involved."