The leader of Turkey's main opposition party on Monday accused the government of being aware of the coup plan before it took place on July 15, drawing a furious response from President Recep Tayyip Erdogan.
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.
"If this list is going to be kept secret then it indicates that July 15 was a controlled coup," said Kilicdaroglu, quoted by NTV television. "They (the authorities) had information about the coup in advance."
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.
There have already been questions over reports that the Turkish MIT intelligence service was aware of the coup plot on the afternoon of July 15, although it only erupted later that night.
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.
"What do you mean by a 'controlled coup'? Is this not an insult to the martyrs and the heroes? He must put the evidence on the table for such claims."
"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.