In a letter to Rod Morgan, chair of the Youth Justice Board, the prison reform campaign group said the revised version of Prison Service Order 4950 would leave under-18s without a guaranteed period of time out of their cells.
Originally, the order said young people need to be out of their cells for 10 hours a day, but this requirement is absent in the still unpublished version.
The league's letter also criticised the removal of a measure requiring prisons to provide 15 hours of education to young people a week from the order.
Frances Crook, director of the Howard League for Penal Reform, said the changes are concerning.
"If they don't specify the time to be spent out of the cells, and there are staff shortages, children could be locked up all day," she added.
She also hit out at the decision to apply the order to both juvenile boys and girls rather than having separate orders as has been done in the past. "Girls have different needs, and there should be recognition of those needs," said Crook.
A spokeswoman for the Youth Justice Board said the order would require prisons to adhere to service level agreements set out by the Youth Justice Board. These agreements, she said, address all the points raised by the league. But Crook said that since these agreements were not public, this was no comfort.
The revised order 4950 is due to be published in September.
