Defendants have attempted to plead guilty even though they claim to be innocent following the introduction of controversial court fees, MPs have been told.
There is evidence that increasing numbers of magistrates are having to reject admissions in such circumstances since the charges were rolled out in April, the Commons Justice Committee heard.
The criminal courts charge, which can range from £150 to £1,200, was introduced by the coalition government to help towards the running of the courts system and is levied on anyone convicted of a crime.
Defendants face larger fines if they are convicted after a trial rather than pleading guilty at an early stage.
Richard Monkhouse, chairman of the Magistrates' Association, was asked about the propensity for people who are innocent pleading guilty as a result of the regime.
He said: "We have some anecdotal evidence that some magistrates have refused to accept an equivocal plea, in other words a plea that is 'I'm not guilty but I'm pleading guilty'.
"That has happened in the past but the frequency of that seems to be increasing."
However, he stressed that it was too soon to draw clear conclusions about any impact because there is no official data yet on how the proportion of pleas has changed.
Mr Monkhouse confirmed that more than 50 magistrates have resigned in protest against the fees.
He said: "As chairman I get a lot of emails and letters from magistrates who have specifically quoted this, maybe not as the only reason, but as the final straw.
"There are some who have said 'no this is it, I cannot bring myself to do this'."
Last month a magistrate in Leicestershire resigned, having served as a JP for 16 years, after offering to pay towards the "foolish" levy when a penniless asylum seeker was ordered to pay £180.
Nigel Allcoat was suspended and an investigation was launched after he attempted to pay part of the convict's costs as a "humanitarian" act.
After quitting he said: "I just can't belong to an organisation that puts people in foolish situations like this. This charge has never been thought through."
Justice Secretary Michael Gove has told MPs he will review the levy introduced by his predecessor after acknowledging "widespread concern" about its operation.
However, officials described reports the charge could be scrapped as simply "speculation".
Malcolm Richardson, deputy chairman of the Magistrates' Association, told the committee the organisation has not taken a position on the principle of the charge.
However, he added: "We certainly wouldn't stand in the way of anyone who wanted to abolish it."