NUS lawyers have written to Business Secretary Sajid Javid, demanding he properly considers the equality impact of the move.
Plans to scrap grants were announced by Chancellor George Osborne in his budget in July.
The government said it would respond to the letter in due course.
Currently, students whose family income is below £25,000 are eligible for full grants worth up to £3,387 plus a loan of £4,047 if they live away from home outside London.
Partial grants on a sliding scale are available for students whose family income is below £60,000.
The government wants to replace the grants with loans for new students from next year.
In its judicial review pre-action letter, the NUS identifies "a serious and unlawful failure" in the government to discharge a duty set out in the 2010 Equality Act to consider the impact of the policy on the poorest students.
"The abolition of maintenance grants will cause a very significant increase in debt for the poorest students; from £40,500 to around £53,000 at a time when the impacts of the 2012 recent reforms remain to be properly analysed and understood," says the letter.
NUS President Megan Dunn called the government's plan "reckless", citing "strong evidence" the change would mean higher education "becomes less accessible to minority groups".
She added: "We know the huge damage that this change will have if it is allowed to happen. It is obvious that the government is attempting to rush through these changes with no consideration on future generations of students."
Salima Budhani of legal firm Bindmans said the NUS was calling upon the secretary of state "to halt plans to abolish maintenance grants and to gather information to enable him to properly consider the complex equality considerations at stake.
"An open minded reconsideration of the policy should then take place."
A spokeswoman for the Department for Business Innovation and Skills said students would not have to pay back anything until their earnings rose above £21,000.
The maintenance loan for all students would also rise from next year to £8,200 for students living away from home outside London: "the highest amount of support ever provided", the spokeswoman added.
The National Union of Students (NUS) has taken the first step in legal action over government plans to scrap maintenance grants in England.