Four thousand extra prison places could be needed if guidelines designed to encourage court defendants to make early guilty pleas are adopted, a parliamentary report warned.
The guideline proposed by the Sentencing Council would make clear that defendants can benefit from a one-third reduction to their sentence only if they plead guilty on their first appearance in court - rather than at the first reasonable opportunity.
And it will reduce from a quarter to one-fifth the discount on offer for pleading guilty at a later stage in proceedings.
The House of Commons Justice Committee warned that the change risks an "adverse impact" on the prison population and may particularly affect vulnerable defendants, including those with learning difficulties and mental health conditions.
In its report, the committee endorsed the goal of encouraging defendants who are going to plead guilty to do so as early as possible.
It said this could "reduce the impact of the crime on victims, save victims and witnesses from having to testify, and save public time and money on investigations and trial".
But it warned that defendants who miss the first deadline for a guilty plea may now opt to go for full jury trial, rather than take the less generous one-fifth reduction on offer.
This could lead to longer sentences, even though the Sentencing Council does not expect any increase in the overall number of convictions as a result of the change, the report warned.
The council has estimated that 1,000 more prison places may be needed even if more defendants take advantage of the one-third discount. And the report said that the number could rise to 4,000 if more opt for jury trial.
Committee chairman Bob Neill said: "There has not been enough research to assess the possible impact on prisons and other aspects of the criminal justice system.
"The Sentencing Council should conduct further research into the factors that influence a defendant's decision to plead guilty, to inform a more comprehensive and robust reassessment of the draft guideline, taking into account costs and savings to all aspects of the criminal justice system, especially the prison population."
The committee also highlighted research showing that more than a quarter (26.7%) of those who apply for a review of their conviction had entered a guilty plea in court.
The Criminal Cases Review Commission gave evidence that people with mental health problems may feel under pressure to plead guilty, even if they are "factually innocent".
Mr Neill said: "We were surprised that the consultation paper does not mention the potential impact of the guideline on disabled defendants - in particular, those with learning disabilities, autism or mental health conditions.
"The council should undertake a comprehensive equality impact analysis, and consider how to mitigate any adverse impacts relating to defendants with protected characteristics including disability."