Gandhi assured him of her party's support to the legislation, which she said would be a significant step forward in the empowerment of women.
The Rajya Sabha had passed the bill on March 9, 2010.
"I am writing to request you to take advantage of your majority in the Lok Sabha to now get the women's reservation bill passed in the Lower House as well," Gandhi said in the letter dated September 20.
Speaking to reporters here, Congress women's wing chief Sushmita Dev questioned the alleged delay by the government in ensuring the passage of the bill in the Lok Sabha.
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The ruling BJP should show that its commitment towards the legislation is not mere "symbolism", she said.
She asked Modi to assure the country's women that the reservations would be implemented before 2019, when the next general elections will be held.
In her letter, Gandhi recalled that the Congress and its late leader Rajiv Gandhi had first mooted the provision for quotas for women in panchayats and municipal bodies through Constitution amendment bills.
Dev, who read the letter out to reporters at the briefing, said, "The question we ask Modiji after three years of the BJP coming to power) is...why the delay?"
The lawmaker said given the work that will follow once a decision is taken to pass the bill in the Lok Sabha and the time that it will take, the government will have to get going with the business "right now".
To a question by a reporter on why the main opposition party did not take the matter up with the government after the BJP came into power, Dev said the issue was flagged on several occasions including during debates on the floor of the House.
Naidu, Dev said, had also stated that the government was "discussing and seriously thinking" about the bill.
"Once a Parliamentary Affairs minister makes that statement on the floor of the Lok Sabha and if Sonia Gandhiji gives him time of six months or a year to implement that, I will say it was her faith in the government's motive. But this did not happen," she said.
Another Congress leader, Shobha Oza, seconded Dev and said the party had collected "lakhs of signatures" from across the country and those would be submitted to President Ram Nath Kovind while pressing for the demand.