Hardik handed the letter in a sealed cover so its content was not known, said Mahesh Savani, one of the mediators.
Savani and another mediator, Mukesh Patel met Hardik at Lajpore jail in Surat.
"After our meeting with the ministers at Gandhinagar, we met Hardik in jail today at Surat. He gave his nod to continue peace talks with government and handed over this letter," he said.
"In this letter, Hardik has asked PAAS members to refrain from making irresponsible statements before the media as it adversely affects the agitation and vitiates peaceful atmosphere in the state. He warned PAAS members not to make any statement which can help BJP or Congress," said Savani.
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Hardik has also named PAAS conveners who are authorised to give statements to the media, he said.
Meanwhile, Congress MLAs Harshad Ribadiya and Mahesh Patel today visited Lalji Patel, president of another agitating Patel outfit, Sardar Patel Group, at a hospital in Mehsana.