Neil Gerrard, MP for Walthamstow, John Grogan, MP for Selby, Katy Clark, MP for North Ayrshire and Arran, and Jon Trickett, MP for Hemsworth, have tabled two amendments to the Offender Management Bill designed to keep what they describe as "core" probation activities within the statutory sector.
Although the majority of the activities listed in the amendments are of little interest to charities seeking to take on more probation contracts, they do include the rehabilitation of offenders - a key area with which many voluntary providers are already involved.
The amendments, which have so far won the support of 18 MPs, 15 of them Labour, have caused alarm in the sector.
"Presumably, rehabilitation includes the education of offenders and things like drugs treatment programmes," said David Chater, head of policy at youth charity Rainer. The charity's activities include working with young offenders and it is interested in expanding its role in probation provision.
"That would take back in-house a huge amount of work currently delivered by the voluntary sector."
Chater is concerned that, if the amendment is voted through during the bill's report stage next Wednesday, the legislation could be undermined entirely.
"I think if those amendments go through, then the bill would be dropped," he said. "They go beyond issues of what the voluntary sector should and shouldn't deliver.
"The wider social market approach of some of those rehabilitation services would effectively be scrapped."
But Gerrard denied that the amendments would have much effect on charities.
He said: "What the amendments protect are court reports, giving assistance to courts in determining sentences, and so on.
"They leave open the sort of work done on education, drugs - all of the stuff a lot of voluntary organisations do now. Nobody has got any problem with that."
The Children's Society has won a Justice Award for its Bristol-based Right Track project, which works with children in trouble with the law.
It is using the government award to call on more councils to partner with the sector on youth justice work. Stephen Connor, director for children and young people at the society, said: "Working with us will help councils to reach their targets and young people to realise their potential."