Dennis said Nigel Stepney, a Ferrari employee since sacked by the team, had tipped off Mike Coughlan, McLaren's chief designer, in March that the Italian team's car was in breach of the regulations.
"Specifically, he told Mr Coughlan about a floor attachment mechanism and a rear wing separator, both of which could be and were seen on the Ferrari car prior to the Australian Grand Prix," he said.
"As far as we are aware, Ferrari ran their cars with this illegal [floor] device at the Australian Grand Prix, which they won," he said. Dennis's side is currently 27 points ahead of Ferrari in the standings.
"In the interests of the sport, McLaren chose not to protest the result of the Australian Grand Prix even though it seems clear that Ferrari had an illegal competitive advantage."
Dennis said Stepney was right to bring the matter out into the open.
"It is in the interests of Formula One that whistle-blowing is encouraged and not discouraged," he said.
However, despite McLaren reportedly instructing Coughlan to cease contact with Stepney, they allegedly met in Barcelona, Spain, on April 28.
"He kept these documents at his home and later with the assistance of his wife who copied [them] on to two CDs at a shop near their home before shredding the originals using a home shredder and burning them in his back garden"
McClaren team boss
In July, two computer discs with Ferrari material were found in a search of Coughlan's house carried out on the orders of the Italian team.
"As it is now in the public domain, Mr Coughlan has admitted that Mr Stepney gave him a dossier of Ferrari documents in Barcelona which he took for his own private reasons," Dennis wrote.
"He kept these documents at his home and later with the assistance of his wife who copied [them] on to two CDs at a shop near their home before shredding the originals using a home shredder and burning them in his back garden."
Dennis said that since that discovery, Ferrari had gone to "extraordinary length to try to maximise the damage to McLaren, no doubt hoping to gain some advantage for the world championship".
He said that no one else at McLaren was aware of the documents or of any Ferrari details finding their way into the design process.
"McLaren's reputation has been unfairly sullied by incorrect press reports from Italy and grossly misleading statements from Ferrari," Dennis said.
"It would be a tragedy if one of the best world championships in years was derailed by the acts of one Ferrari and one McLaren employee acting for their own purposes wholly unconnected with Ferrari or McLaren."