Four in 10 people sent to crown court to be sentenced could have been dealt with by magistrates, wasting valuable time and money, the justice minister will say.
In a speech to magistrates, Damian Green will say more must be done to make the best use of them and to keep the right cases in the right court.
The speech is the first of four "pre-consultation" events with magistrates ahead of public consultation on their role set to start later this year.
The work is part of a wider overhaul of the criminal justice system, and will include unclogging magistrates' courts by dealing with simple road traffic offences out of the traditional process, as well as tackling reoffending rates with planned new powers for magistrates.
Ahead of the speech, Mr Green said: "Magistrates are truly the cornerstone of our justice system; not only that, they are a model of what a good citizen should be.
"They volunteer to give their skills, expertise and time for the good of others, for nothing. We are lucky to have them, and we should be proud of them.
"However, four out of 10 defendants sent to the crown court for sentencing received custodial sentences that could have been handed in the magistrates' court - we need to look at why this is happening and if we need to do more to make the best use of magistrates. We need to keep the right cases in the right court if we are to have a modern justice system in a fair society."
There are around 23,500 magistrates in England and Wales, who sit across adult, youth and family courts. They have the power to sentence criminals to up to six months in prison and issue fines of up to £5,000. Last year around 9,800 defendants were convicted by magistrates then committed to crown court for custodial sentences.
But 40% of those received up to six months in prison, meaning they could have been dealt with in magistrates' court where the cost of a typical sitting day is around £1,400 per day, compared to £2,150 in the crown court.
Similarly, around 3,200 defendants under the age of 18 were also committed to the crown court for trial in 2012, despite the fact the youth court - set up specifically to deal with children involved in criminal proceedings - has the power to impose a Detention and Training Order, served half in custody, half in the community, up to a maximum of 24 months.