They said such a step "would be a violation of the Forest Rights Act and amount to violating its central provisions".
The letter has been signed by former Delhi High Court Chief Justice Justice A P Shah, retired Bombay High Court judges Hospet Suresh and Kolse Patil, former Calcutta High Court judge D K Basu, Supreme Court lawyer Vrinda Grover and former UN official Miloon Kothari.
They noted that the Forest Rights Act and the Forest (Conservation) Act apply to different but overlapping areas of law - the former is concerned with forest rights and the latter with diversion of forest land.
"There is a common misconception that the lack of explicit reference to diversion of forest land for large projects in the FRA means that this Act can be ignored when diverting land for such purposes. This is incorrect," they said.
"No statute covers every subject matter that is related to it. Rather, when two laws cover related areas, it is necessary to harmoniously construct them in order to ensure that the objectives of both are achieved," they said adding that the law must be read in such a fashion as to enhance the welfare of the concerned groups of people, and not to damage them.
"...Obtaining a certificate from the gram sabha regarding the completion of rights recognition and consent to diversion is the minimum that is required for the Forest Rights Act to be respected during the process of diversion of forest land," the letter said and warned that any move to dilute or undermine this would be in violation of law.