Lawyers will be given financial incentives to encourage clients to plead guilty early under government reforms to legal aid but will lose money if cases go to trial, according to solicitors in London.

The warning emerged during the last day of the Ministry of Justice's consultation on saving £220m from the annual criminal legal aid budget and as former Liberal Democrats prepared to protest outside the party's headquarters against coalition support for the policy.

The London Criminal Courts Solicitors Association (LCCSA), which has examined revised fee figures in the MoJ consultation, says that a pattern of perverse financial incentives will affect both magistrate and crown court cases.

"A client pleading guilty to a standard actual bodily harm charge in crown court will earn their lawyer as much as a 20% fee increase," the LCCSA said. "There are some cases in the crown court where a quick guilty plea will earn a lawyer a 75% fee increase.

"Likewise, in magistrates courts a simple guilty plea [for instance, for common assault] will reward lawyers with a 17% pay increase. This flies in the face of the government's advertised 17.5% cuts to save £220m from the legal aid budget."

The association said the revised fees would result in some lawyers losing out as much as 65% in some magistrate court cases and up to 73% in some crown court cases. Steven Bird, a London solicitor and LCCSA member, said: "The only conclusion to draw from these figures is the sad truth that the new fee structure is ideological and has nothing to do with austerity.

"By law, we're already obliged to advise our clients about the benefits of an early guilty plea, by way of credit on their sentence … It doesn't take a legal background – or criminal record – to realise that these incentives for a guilty plea and disincentives for a trial are an affront to justice."

Responding to the claim, the MoJ said: "For a lawyer to advise a client to plead guilty when they are not would be one of the most serious breaches of the profession's code of conduct, and could see them lose their right to practise. We do not believe a professional lawyer would do so.

"At around £2bn a year we have one of the most expensive legal aid systems in the world and even after our changes would still have one of the most generous. We cannot avoid finding efficiencies to ensure it remains sustainable and available to those most in need of a lawyer."

The MoJ said the consultation involved average cuts of 17.5% and the existing "overly-complex fee structure" needed reform. The department has now told lawyers that cuts of up to 30% in so-called very high cost cases will start from 2 December. Some lawyers have threatened to boycott cases if fees are cut.

Meanwhile, the Justice Alliance, which is opposed to the legal aid cuts, is planning to demonstrate outside the Liberal Democrats headquarters on Friday morning. Among them will be Professor Philippe Sands QC, Dinah Rose QC and Jo Shaw, who recently resigned from the party.

Rose said: "The protection of the right of access to justice ought to be fundamental to a party which values civil liberties. It is put in jeopardy by the government's proposals to implement yet more cuts on legal aid."

Matt Foot, a solicitor and founder of Justice Alliance, said: "The government has no idea whether the proposals will work or whether it will cause total chaos. MPs from across the political spectrum have expressed their concern at government plans which will make it hard for ordinary people to challenge the state."

The Bar Council, which represents barristers in England and Wales, also released a statement opposing the cuts. Maura McGowan QC, chairman of the Bar, said: "What we have seen … is the denigration of thousands of members of the profession, who work hard in the public interest, whether in civil or criminal courts, and have had to endure deeper cuts than anywhere else in the public sector."

The Law Society, which represents solicitors in England and Wales, although less critical, also opposed the proposal. The society's president, Nicholas Fluck, said: "We are confident that, with some modifications, the Ministry of Justice's proposals can ensure that anyone accused of a crime and unable to meet the costs of legal representation has access to a high-quality defence solicitor of their choosing."

"It is unwise to risk tarnishing the respect in which our justice system both domestically and internationally is held by an apparent incentive to plead guilty, which could jeopardise the relationship of trust between clients and solicitors."