A point of dispute between New Delhi and Islamabad, Sir Creek runs along Gujarat's border with Pakistan.
Mr Modi has in a letter to the Prime Minister said, "I would earnestly request you to stop this dialogue with Pakistan at once and Sir Creek should not be handed over to Pakistan." (Read full letter)
The Prime Minister's Office retorted, " Shri Modi has written his letter without making any efforts to ascertain the facts from the Government of India." (Read PMO's response)
At a press conference today, Mr Modi's party, the BJP, disclosed the details of Mr Modi's letter and claimed that because he is in the midst of an election, the chief minister followed protocol and signed the letter as "a concerned citizen and not as a chief minister of a state."
The Prime Minister's Office differed, stating, "The writing and release of this baseless letter by the Chief Minister of Gujarat in his "personal" capacity, a day before elections in the state, is mischievous."
Gujarat votes today and on Monday; results of the election will be declared on December 20.