Plans to cut the amount of money spent on legal aid and stop criminal trials over-running in England and Wales are to be unveiled by the Lord Chancellor.
Lord Falconer said the reforms would be an 'absolute priority'
Under the proposals, lawyers would bid for legal aid contracts and they would have to foot the bill if a criminal case lasted longer than expected.
Since Labour came to power eight years ago, the legal aid bill has increased by more than a third to over £2bn.
But critics fear the plans could lead to "rushed justice".
The Bar Council said it was wrong to suggest lawyers exploited the system.
Chairman Guy Mansfield QC said the current system for paying barristers in criminal cases was "antiquated" and based on pay structures agreed 10 years ago.
LENGTHY CRIMINAL TRIALS
The Jubilee Line fraud trial collapsed in March after 21 months and cost £60m
The Blue Arrow fraud trial of 1992 cost £40m and lasted for twelve months
The trial of Ian and Kevin Maxwell in 1996 lasted eight months and cost over £25m
He said the system failed to pay barristers adequately for preparatory work that shortens the length of trials.
The Lord Chancellor told BBC News the proposed changes were "not an attack on lawyers" but an attempt to "make sure that the money in legal aid is focused where it should be".
Lord Falconer, whose proposals are set to be detailed in a new strategy document, said most cases should not last longer than "a few months at the very, very most".
He said last year around 1, 300 crown court cases - which accounted for around 1% of criminal cases - swallowed up 50% of legal aid expenditure, with just 13 cases costing £48m between them.
Under the proposals, the money saved on criminal trials would go instead to civil legal aid, such as family hearings.
Trials would be made more efficient by making prosecution and defence teams agree more areas of a case before going to court, Lord Falconer added.
But the proposals have faced a number of criticisms.
Liberal Democrat home affairs spokesman Mark Oaten said: "Legal cases should be conducted on the basis of thorough process, not clock-watching."
He added: "Rushed justice may not be fair justice."
And Roger Smith, director of civil rights and law reform group Justice, warned: "Lawyers will underbid to get a contract, cut their resources to the minimum and then seek to exploit their position as one of a severely limited number of contractors."
Meanwhile, Mr Mansfield said the plans would "decimate the young Bar and our already hard-pressed network of community-based high street solicitors."
But the Law Centres Federation, which represents organisations offering free legal advice, cautiously welcomed the proposals.
Director Steve Hynes said he hoped it would address the "civil legal aid crisis" which he claims has seen vulnerable people turned away from law centres due to a lack of resources.