Irwin Mitchell, the UK's largest legal aid practice, has written to minister of state Geoff Hoon, warning that the draft funding code may breach the human rights convention and create an unworkable new bureaucracy.
Partner Andrew Lockley says: "Government cuts on legal aid will be even worse than has hitherto been realised and are just the tip of the iceberg."
The Lord Chancellor has claimed support for his reforms in the past by quoting leading civil firms such as Thompsons.
Irwin Mitchell's study of the code for funding individuals highlights four key concerns:
l Having to show that the case will be of benefit to more than just the individual will restrict the rights of vulnerable people, such as prisoners and disabled people, to judicial review.
2 Only a minority of clinical negligence cases covered by the existing scheme would continue to be helped at the investigative stage.
3 The code assumes that solicitors will take group actions on a no win, no fee basis, but judging by the recent failed tobacco litigation, Irwin Mitchell does not believe that no win, no fee agreements will be popular.
4 Legal aid could be refused and deserving cases abandoned solely because a budget has been used up. That could breach the human rights convention and expose the Legal Aid Board to judicial review.
Lockley says that under the code most cases will have to pass a 17-step test to qualify for legal aid, creating a bureaucratic nightmare.
"It is so complex that many solicitors will steer clear of such an administrative burden and, consequently, further restrict the legal aid outlets available."