A magistrate who resigned in disgust at criminal courts charges being imposed on defendants who plead or are found guilty has told the Guardian the fine “disproportionately punishes the poorest in our community”.

Marie Lewis, who was a magistrate for seven years before she resigned earlier this month, said she loved her unpaid vocation but could no longer levy the charge she believes pressurises defendants into pleading guilty for fear of incurring excessive costs.

“Over the years I have served, I have demonstrated that I can make difficult decisions when required,” Lewis has written in a letter to the Guardian. “I have sent single mothers to prison, removed children from their parents and destroyed dangerous dogs let down by their owners. I can do difficult things when they are just and proportionate.”

But the arrival of the criminal courts charge, which requires those who plead guilty at a magistrates court to pay £150 but forces those convicted at crown court to pay £1,200, poisoned her experience of dispensing justice.

The qualified nurse and former justice of the peace, from Holmfirth, West Yorkshire, is the latest of more than 50 magistrates in England and Wales who have quit in protest at mandatory charges introduced earlier this year by the former justice secretary Chris Grayling.

“The criminal courts charge disproportionately punishes the poorest in our community,” she said. “Those who cannot pay for a TV licence, a train ticket, or previous fines. Simultaneously, the job centres are sanctioning people’s benefits lasting for anything up to three years without a fair trial.

“These charges undermine the right to a fair trial, putting pressure on people to plead guilty for fear of the costs. They limit our ability to impose fines, prosecution costs and rightful compensation. They have reduced the discretion magistrates should possess to sentence based on individual circumstances and, finally, I believe they are unworkable and will cost more to recover [and remit] than they are worth.”

She said the the final straw was a relatively minor case involving a woman who left court after pleading guilty with outstanding payments and fines of £1,500, which she had been ordered to repay at £5 a week.

“It will take her more than five years to repay that and because we could not fine her any more we had to give her a curfew order. I felt like we were raising a stealth tax for the government. We couldn’t give an appropriate sentence to this woman. I came home and I cried.”

Lewis, who until recently worked as a psychology lecturer, said she does not think publicising her departure will bring the magistracy into disrepute, and added: “There will be more and more leaving.”

Lewis’s protest resignation came shortly before a motion of regret calling for the criminal courts charge to be abandoned was debated in the House of Lords. Opponents of the mandatory charge defeated the government by 132 to 100 votes – although their moral victory has no force to change the law.

The Labour justice spokesman, Lord Beecham, who proposed the motion, said: “The uniform imposition of these fixed charges is contrary to the courts’ current approach, which is one of totality – taking into consideration the nature of the offence and the effect, including the financial effect of fines and costs already levied.

“Judicial discretion under these regulations is being displaced by what one might call Ryanair justice, with significant add-ons, often disproportionate to the basic financial penalty.”

Last week the lord chief justice, Lord Thomas of Cwmgiedd, said that steep rises in criminal and civil court fees were putting access to justice beyond the reach of most people and “imperilling a core principle of Magna Carta”.

Richard Monkhouse, national chairman of the Magistrates Association, the independent charity representing magistrates in England and Wales, said: “We always lament the loss of talented and experienced magistrates from the bench through resignation, but our message to those considering resigning is to have faith, because we believe the case for addressing the charge is overwhelming and that it’s being heard by [the current justice secretary] Mr Gove.

“The mood music appears to be softening and our members welcome this. We hope that Mr Gove will heed our calls and grant magistrates discretion over the charge, after all, his track record in the job indicates that he is open to new ideas.”

A Ministry of Justice spokesman said hopes that the criminal courts charge would be altered were speculation. He added: “It is right that convicted adult offenders who use our criminal courts should pay towards the cost of running them.

“The introduction of this charge makes it possible to recover some of the costs of the criminal courts from these offenders, therefore reducing the burden on taxpayers.

“Instalment payments can be set up if a defendant’s means do not allow prompt payment in full. This will allow offenders to pay the charge in affordable instalments.”

This week Michael Gove reversed several previous MoJ policies pursued by Grayling, including a planned prison contract with Saudi Arabia and proposals to contract out to private firms the collection of court fines.

