A "cornerstone" of the legal system, the universal right to a solicitor upon arrest, could be jettisoned in favour of means-testing under controversial plans drawn up by the Ministry of Justice.

Legal experts including Lord Ken Macdonald QC, a former director of public prosecutions, have expressed alarm at the proposal and questioned how it would work in practice.

Legal aid charities said the new measure, which was quietly slipped into the legal aid, sentencing and punishing of offenders bill, had taken them completely by surprise and predicted that it would be subject to fierce debate in parliament.

The proposal would effectively set a bar that would see people who are earning above a certain amount of money or who have access to a particular level of savings forced to pay for legal representation.

"There are issues of principle here," said Richard Miller, head of legal aid policy at the Law Society. "When someone is arrested they are in the power of the state, subject to the mercies of the police officers involved.

"The purpose of having a solicitor acting for them is to ensure their rights are respected, that they are not physically abused, that their confessions are not forged and they are not detained for longer than legally allowed.

"It's been a cornerstone of our justice system for the last 25 years and the idea that it should be changed is entirely wrong."

The universal right to representation by a solicitor at a police station was enshrined in the 1984 Police and Criminal Evidence Act (Pace) in response to a series of serious miscarriages of justice in the 1970s and 80s involving unrepresented defendants.

However, the government, which has already signalled its determination to cut some £350m from the legal aid budget, is keen to look for further savings. While a spokeswoman for the Ministry of Justice said it currently had "no plans" to introduce means-testing for legal representation upon arrest, she confirmed the new clause would give the government the power to introduce it if circumstances dictated. "The provision in the bill to permit means-testing ensures future flexibility over the operation of the legal aid system should it be needed," the spokeswoman said.

The confirmation drew a sharp response from Macdonald. "This is a critical part of the apparatus of protection that we have," he said. "The presence of a lawyer doesn't just protect the defendant from police, it protects the police from a defendant making up allegations about what happened, for instance during the course of an interrogation. I think the government should be very cautious about interfering in any way with the absolute right to representation in police stations. It's there for a very good reason. When we didn't have it ,we saw the consequences."

Legal bodies are concerned that the government plans to fast-track the legislation, which they claim will disadvantage the poor, through parliament.

They complain that a second reading of the bill has been scheduled for Wednesday, which allows little time for adequate scrutiny of the details of the legislation.

The justice secretary, Kenneth Clarke, insists that urgent cuts are necessary because England and Wales have "the most expensive legal aid system in the world". However, Sound off for Justice, a campaign run by the Law Society, claims the figures are misleading and has outlined alternative ways to save money.

Macdonald questioned whether the government's plans would save the taxpayer money in the long run. "Often it is more expensive to administer means-testing than operate it. Who's going to ask you what your earnings are or how much your mortgage is? We are talking about having to ask these questions in stressful situations. The whole thing would be very unwieldy."