Vulnerable people struggle to get legal aid and planned reforms will make this worse, the Law Society has warned.
Parents fighting to stop children being taken into care and domestic abuse victims are among those who will be unable to find solicitors, it claims.
It has launched a campaign 'What Price Justice' backed by 19 groups, including Mind, Shelter and Citizens Advice.
Lord Carter Coles, who put forward the reforms for England and Wales, says they will improve legal aid provision.
The plans would see lawyers bid competitively for all legal aid work, instead of working for set hour fees.
Law Society chief executive Desmond Hudson said low pay rates already made it impossible for some solicitors to run a practice on legal aid fees.
"Getting legal aid advice is a postcode lottery now but these plans could turn it into a national lottery"
"Fewer and fewer solicitors are able to undertake that work," he said. "Even fewer solicitors, we fear, may be able to undertake that work under the government plans."
He said it meant people "in large parts of the country" would find it "very difficult" to find a solicitor for legal aid work.
"Getting legal aid advice is a postcode lottery now but these plans could turn it into a national lottery."
The Law Society said criminal legal aid solicitors last received a pay rise in 2001 and civil legal aid solicitors only received one pay rise in the last five years - 2.5% in 2004.
It said 95% of the 436 civil legal practitioners who responded to its online surveys believed the introduction of fixed fees would mean legal aid work was not viable.
'Marketisation'
It has estimated that as many as 800 law practices could close as a result of the Carter Review proposals - equivalent to a quarter of all current providers of legal aid advice.
The government's reforms are designed to tackle this shortage, but carry a 5% cut in the £2bn budget.
Lord Carter says his reforms were the most effective way at tackling the problem. He has suggested a "marketisation of legal aid" driven by competition based on quality, capacity and price.
The Law Society wants the government to invest in stabilising the legal aid sector in advance of any such reforms.
It says the new business structures proposed "could be made to work" but only with proper investment.
The Department for Constitutional Affairs said a decision on the proposals would be made "in due course".
A spokeswoman said: "We consulted widely and are now looking at the responses."
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