New rules designed to stop high earners like Premiership footballers from claiming legal aid are in effect from today.
But many solicitors fear the new system is overly bureaucratic and will slow down the criminal justice system.
The changes mean anybody charged with a criminal offence and asking for financial help in a magistrates' court will qualify only if they cannot afford a lawyer.
Legal Aid Minister Vera Baird QC MP said: "It is right that those who can afford to pay for their own defence should do so, not the taxpayer.
"The new measures will ensure that money is focused on the most vulnerable who cannot afford to pay for a lawyer."
A spokesman for the Department for Constitutional Affairs said: "The introduction of a simple financial test will put an end to legal aid being granted to high earners including Premier-ship footballers."
However critics say there is widespread confusion about how the new system will operate, and believe the 26-page application form for legal aid will take too long to fill in.
Richard Miller, director of the Legal Aid Practitioners' Group, which represents 600 law firms, said: "There is confusion across the whole country about how it is supposed to work." He added: "There is a real concern that this is going to be a very bureaucratic process. Cases are going to be delayed, it is inevitable."
Desmond Hudson, chief executive of the Law Society, said he had written to Ms Baird expressing his concerns.
He said: "It's right that defendants who can afford to pay their own legal costs should be made to do so once convicted.
"But it is unrealistic to expect the means test form to be completed and submitted along with all the supporting documentation within just 48 hours of a suspect being charged."
Means testing is being re-introduced after it was abolished five years ago following concerns that it was overly bureaucratic and expensive.
But critics fear the new system may see a return to the same problems despite pledges that lessons had been learned.
Mr Miller said there were also concerns over whether an electronic link between court and a central benefits database, to prove applicants' eligibility once they have given their National Insurance numbers, would work.
A DCA spokesman said that the system had been designed "to be as simple as possible" and in many cases "applying will be very straightforward".