The Lord Chief Justice, Lord Bingham questioned whether the move to end legal aid for most personal injury cases and replace it with "no win, no fee" agreements with lawyers had been properly thought through.
The Lord Chancellor, Lord Irvine, is in favour of the no win, no fee deals as a means of cutting the legal aid budget.
Under the new scheme a solicitor would agree to take no fee if they lose in return for a higher fee if they win. But their clients have to take out legal insurance to cover the risk of having to pay the other side's fees if they lose. Lord Bingham said he was "unsure" whether private insurance schemes would step into the gap and provide affordable cover.
He warned that the Government must ensure justice remained available to all: "One cannot read the well-argued responses to the Government's proposals ... without appreciating the difficulties and the potentially fateful consequences of any radical change to the current arrangements."
Giving the Barnett Lecture at Toynbee Hall in the East End of London, Lord Bingham said: "Those of us who have had no contact with insurance interests cannot be other than unsure whether insurance cover will be available in the much wider range of civil proceedings which are suggested as suitable for conditional fee agreements.
"If such cover is not generally available at reasonable cost, then it seems unlikely that conditional fee agreements will fill the gap left by the withdrawal of legal aid."
Speaking on the centenary of the Poor Man's Lawyer scheme - a pioneering free legal advice service - Lord Bingham said that the Government's proposals had to maintain the principle that "the laws of our country exist for the benefit of the poor as well as the rich; that equality before the law is a pretence if some citizens can assert and protect their rights and others cannot; that the rule of law, to be meaningful, must ensure that justice is available to all."
Lord Bingham said all sides in the debate accepted the need for reform of the legal aid system because it had become "hugely and uncontrollably expensive". A budget of pounds 682m in 1990-1 had more than doubled in six years to almost pounds 1.5bn.
The cost of each action had risen by more than the rate of inflation and the number of people helped by the scheme had fallen.
Heather Hallett QC, chairman of the Bar Council, which represents barristers, said her organisation shared the Lord Chief Justice's concerns. "We hope the Government will listen to these concerns and proceed very carefully with change. We must not take a leap into the unknown leaving the less well off paying the price."