Sir Anthony Hooper said judges across England and Wales were "extremely worried? about proposals to auction off criminal defence work to the lowest bidder, which could see the system run by corporations like G4S and Eddie Stobart.
Speaking publicly for the first time since he retired in September, Sir Anthony said: "It is very difficult for serving judges to be critical of Government policies but I can tell you I have spoken with them and they are extremely concerned.
"The purpose of our criminal justice system is to acquit the innocent and convict the guilty. This requires a competent prosecutor, competent defence advocate and competent judge. If you take any of these elements away, the results will be costly and potentially disastrous, with innocent people being convicted and potentially dangerous individuals wrongly acquitted.?
Under the Transforming Legal Aid proposals, Justice Secretary Chris Grayling aims to shave £220million from the £2billion legal aid budget by tendering contracts to 400 firms and mega law shops around the country.
The proposals, which are under consultation, will see people accused of crimes and who cannot afford their own lawyers forced to accept a legal adviser appointed to them, rather than choosing a legal aid solicitor they know and trust.
Those legal advisers will receive single fixed fees to represent clients, whether they plead guilty or not, raising fears that there will be little incentive to conduct a defence properly.
"I?m afraid we are abandoning quality and replacing it with the lowest bid,? said Sir Anthony last night.
"We have already seen what has happened when the Government appointed a single company to provide all the interpreters for courts around the country. It has not worked.
"Now specialist solicitor firms who deal with complex baby-shaking cases and fraud cases will no longer be available to serve a client on legal aid because he will be bound to one of a handful of providers in his area who may have no experience of this kind of sophisticated criminal work.?
He also criticised the "conflict of interest? arising between the financial interests of the company in obtaining a cheap and speedy result and those of the client, "which are quite different?.
He added: "We have a huge number of rules in our society which govern conflicts of interest. Ministers have to sell certain shares, MPs have to declare special associations. But here this Government is saying there can be these conflicts of interest.?
Sir Anthony, whose nephew Tom won acclaim as director of blockbuster The King?s Speech, rejected Ministry of Justice comparisons to legal aid fees in Europe.
"By the time a case comes to court on the Continent it is much more likely that the person is guilty because the case has been fully investigated by an investigating judge who weighed up both sides.
"Our system is different. There is still a huge amount of work that needs to be done by the defence.?
Sir Anthony added: "These reforms will be absolutely devastating for the justice system as we know it. They will lead to many problems, certainly to miscarriages of justice. You will see more appeals in the longer term.?