Moves to axe juries in serious and complex fraud trials will erode an essential safeguard of the justice system, MPs and peers have been warned.
Judges alone could decide on 15-20 fraud cases a year
Parliamentarians were urged to oppose the government's plans by barristers' and solicitors' professional bodies, and the civil rights group Justice.
Attorney General Lord Goldsmith QC has unveiled plans to allow a judge to sit alone on complex fraud trials.
The plans, opposed by Tories and Lib Dems, will be voted on in the autumn.
The government proposals follow the collapse of a £60m fraud case after almost two years in court, partly due to jury problems.
The trial of six men, accused of conspiring to corrupt public officials and gain insider information on a £2bn extension to London Underground's Jubilee Line, was abandoned after being blighted by illness among jurors and legal delays.
The government wants judges alone to decide such cases.
But Law Society president Edward Nally said: "Juries provide an essential safeguard in our justice system and the erosion of such a fundamental principle must be stopped.
"It is not the fault of juries that we had fiascos like the Jubilee Line fraud trial.
"The real answer is better case management to prevent trials from being too long and complex."
He added: "The new criminal procedure rules and the judicial protocol for the control and management of heavy fraud cases, which are aimed at reducing the length of complex trials, should be given a chance to work before any decision is taken to abolish juries in fraud cases."
'Not a cost-cutting exercise'
A Bar Council spokesman also addressed the situation at a special meeting of MPs and peers in the House of Commons.
Last month Lord Goldsmith unveiled plans to allow a single judge to sit on complex fraud trails, forsaking the 800-year-old tradition that defendants are tried by a panel of 12 jurors.
The plans enact a change in the law which was passed by Parliament more than 18 months ago.
Ministers will attempt to introduce the law change in the autumn with votes in both the Commons and the Lords.
Lord Goldsmith suggested the measure would be used to handle between 15 and 20 cases in England and Wales each year.
He insisted the aim was not to cut costs, but argued that it was in the interests of justice.
Lord Goldsmith said some trials were currently too lengthy, too expensive and the jurors available for them were not representative of the population.
He was also worried some fraud charges would have to be dropped because they were too complicated.
But civil liberties' groups say that predicted complexity was an excuse which might be used in a host of trials.
The Conservatives say juries are an "essential mechanism" for ensuring the criminal justice system is seen to be fair.
The Liberal Democrats say Labour seems to be prepared "to abandon the principle of fair trials and justice that have served this country well for decades".