Kemal Kilicdaroglu, head of the Republican People's Party (CHP), said he had evidence that the putsch was a "controlled coup" which the government had then allowed to go ahead and later exploit.
Kilicdaroglu told Turkish reporters in Istanbul that up to 180 people within the government had been using an encrypted messaging system for plotting the coup, adding the secret service had a list of these individuals.
The CHP leader said he had "compiled a special dossier" on the issue but declined to be drawn further on its contents.
Tensions are running high in Turkey ahead of the April 16 referendum on expanding Erdogan's presidential powers, with commentators predicting a tight contest.
Ankara says last year's coup was a brutal attempt to seize power by supporters of the US-based preacher Fethullah Gulen that was put down when ordinary Turks surged into the streets to support Erdogan.
This is the first time that Kilicdaroglu, who backed Erdogan after the coup but opposes his plan for an executive presidency system, has cast doubt on the official narrative of events.
"If you have a dossier then out with it!" retorted Erdogan at a televised rally in the Black Sea region of Rize. "But it's a big lie," he added.
Prime Minister Binali Yildirim said the comments were an "insult" to the memory of the 249 people who were killed in the coup.
"No-one can insult this nation," he added.
The narrative of the July 15 coup has become an issue of major sensitivity in Turkey where the failed putsch is seen as a watershed event in its modern history.
Last month, Turkey reacted with fury when the head of German intelligence said Berlin was unconvinced that Gulen was behind the coup.
Disclaimer: No Business Standard Journalist was involved in creation of this content
You’ve reached your limit of {{free_limit}} free articles this month.
Subscribe now for unlimited access.
Already subscribed? Log in
Subscribe to read the full story →
Smart Quarterly
₹900
3 Months
₹300/Month
Smart Essential
₹2,700
1 Year
₹225/Month
Super Saver
₹3,900
2 Years
₹162/Month
Renews automatically, cancel anytime
Here’s what’s included in our digital subscription plans
Exclusive premium stories online
Over 30 premium stories daily, handpicked by our editors


Complimentary Access to The New York Times
News, Games, Cooking, Audio, Wirecutter & The Athletic
Business Standard Epaper
Digital replica of our daily newspaper — with options to read, save, and share


Curated Newsletters
Insights on markets, finance, politics, tech, and more delivered to your inbox
Market Analysis & Investment Insights
In-depth market analysis & insights with access to The Smart Investor


Archives
Repository of articles and publications dating back to 1997
Ad-free Reading
Uninterrupted reading experience with no advertisements


Seamless Access Across All Devices
Access Business Standard across devices — mobile, tablet, or PC, via web or app
