Leading City law firms have launched a stinging attack on a controversial government plan to cut the ?2bn annual legal aid bill by introducing market-style reforms.

In a letter sent on Friday to Lord Falconer, the Lord Chancellor, 28 firms - including big names such as Clifford Chance, Herbert Smith and Lovells - warned that the proposals were a "real threat to access to justice for some of the most vulnerable people in society".

The Law Society-backed intervention is striking because the signatories do no legal aid work themselves, but are weighing in because they think pro bono social law work that they do undertake is plugging gaps already left by legal aid funding shortages.

Michael Smyth, a Clifford Chance partner and chairman of a new group, known as the Social Welfare Law Coalition, said there were already "advice deserts" up and down the country where people had no access to legal services. "There are big provincial centres where there is nowhere offering any social welfare legal advice," he said. "That is not in the public interest."

Lord Falconer warned in July that the legal aid system would get "out of control" unless the government put in place reforms such as the introduction of competitive tendering and the abolition of hourly billing.

The government argues the changes are needed to consolidate and improve the efficiency of a system in which costs have risen by a third since 1997 and ?7m of business is spread among 400 firms. Final proposals are due to be implemented over three years, starting in April 2007.

The law firms argue in their letter that, although the system needs improving and a market model "could be made to work", the current proposals mean it would not make commercial sense for solicitors to take on legal aid business.

The attack echoes criticisms by the London Criminal Courts Solicitors' Association, which has called the proposals a "disaster" likely to force thousands of small firms to close and "seriously damage" the quality of legal advice offered to the public.