New rules designed to stop high earners claiming legal aid in England and Wales are expected to save £35m a year, says the government.
The rules are to be extended to crown courts by the end of 2007
The rules, now in force, mean those facing criminal charges in magistrates' courts only qualify for financial help if they cannot afford a lawyer.
But critics say the new system causes much confusion and will delay cases.
Last year, Premiership striker El-Hadji Diouf was awarded legal aid to fight a spitting charge, sparking outrage.
And last month, multi-millionaire businessman Gerald Smith, who received legal aid, was jailed for eight years for fraud.
Legal Aid Minister Vera Baird said it was "right" that those who could afford to pay for their defence should do so, rather than the taxpayer.
"The new measures will ensure that money is focused on the most vulnerable who cannot afford to pay for a lawyer," she said.
The government expects to save £35m a year from the system's introduction in magistrates' courts. The rules will be extended to crown courts by the end of 2007.
A Department for Constitutional Affairs (DCA) spokesman said: "The introduction of a simple financial test will put an end to legal aid being granted to high earners, including Premiership footballers."
However, 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."
Critics also say the 26-page legal aid application form will take too long to complete.
Mr Miller 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."
And Law Society Chief Executive Desmond Hudson said he had written to Ms Baird about his concerns.
"It's right that defendants who can afford to pay their own legal costs should be made to do so once convicted," he said.
"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."
He said in "countless cases" the solicitor may not have been instructed within 48 hours.
"This unnecessarily complex and bureaucratic system could cause significant disruption," he added.
Means-testing was abolished five years ago following concerns it was too bureaucratic and costly. The DCA insisted the new system had been designed to be "as simple as possible".
The department also said in the vast majority of cases, privately-funded defendants who were cleared would be able to recover some or all of their legal costs.
People who failed a means test can apply for a hardship fund if they cannot pay for "exceptional reasons".