In separate letters to the Gazette, signed by all members of the criminal panel and the vast majority of the family panel, peer reviewers said the rates on offer would drive down standards and make it harder for the most vulnerable clients to have access to good quality advice.
They said the existence of peer review alone was not a sufficient mechanism to ensure quality, as the government has frequently sought to suggest, but that adequate funding was needed as well.
Nick Aspley, a partner at Nottingham firm Marchants and member of the family peer review panel, called for the timetable for introducing new family fees, which are scheduled to be implemented in October, to be suspended and a pilot of the scheme carried out.
?Even at this late stage the LSC needs to get real before the legal aid system collapses,? he said.
Andrew Keogh, a partner at national firm Tuckers and a member of the criminal peer review panel, said: ?Either the rates need to go up or the standards will have to go down.?
He warned that if things remained as they were, firms may end up failing their peer reviews and could ultimately have their contracts terminated.
Law Society Vice-President Andrew Holroyd said: ?This is a very significant development. The experts who the LSC have appointed to check quality of work have confirmed that the fee rates proposed by the LSC will inevitably drive down quality to an unacceptable degree.
?The LSC must urgently rethink its proposals to ensure that legal aid does not become a third-rate service, unable to provide effective access to justice for those who need it.?
But a spokeswoman for the LSC said it was confident that quality levels would be maintained, particularly once firms adapted to working in a fixed-fee environment.
?Higher quality is often no more expensive and in many cases higher quality costs less than average quality, as shown in the peer review data published in the regulatory impact assessment that accompanied Legal aid reform: the way ahead,? she added.