In his letter, Dennis refers to sacked Ferrari employee Nigel Stepney as a 'whistleblower' who alerted chief designer Mike Coughlan to illegal floors and rear wings on the 2007 Ferrari.
"McLaren took steps to confirm whether (Stepney's) allegations were true, and we concluded that they were," Dennis wrote, accusing Ferrari of then winning the Australian grand prix with the illegal floor still attached.
He added: "Were it not for Mr Stepney drawing this illegal device to the attention of McLaren, and McLaren drawing it to the attention of the FIA, there is every reason to suppose that Ferrari would have continued to race with an illegal car."
Dennis slammed as "completely misleading" Ferrari's claim that Stepney's whistle-blowing amounted to the divulgence of "confidential information".
"He acted properly and in the interests of the sport in blowing the whistle about this," Dennis said of Stepney.
He added: "No team can expect their employees to keep quiet if they suspect - correctly in this case - that their employers are breaching the rules of the sport."
Dennis also defended not telling Ferrari about Stepney because "it is in the interests of formula one that whistle-blowing is encouraged and not discouraged".
"If team members think that their identity will be revealed they will not whistle-blow," he wrote.
As for the separate issue of Coughlan's receipt of 780-pages of secret material, Dennis said Ferrari "has gone to extraordinary lengths to try to maximise the damage to McLaren".
He describes as "offensive and false" Ferrari's claim that other McLaren staff were aware that Coughlan was in possession of the Ferrari dossier, which the now suspended chief designer subsequently copied, shredded and burned in his back garden.
Dennis also rejects Ferrari's claim that it was not allowed to participate in the World Council hearing, revealing that Ferrari submitted a dossier of evidence, attended the meeting in Paris with lawyers who were "given several opportunities to ask questions and make submissions", while Jean Todt also spoke.
He said: "McLaren's reputation has been unfairly sullied by incorrect press reports from Italy and grossly misleading statements from Ferrari.
"We believe that the Ferrari press releases, the leaks to the Italian press and recent events have been damaging to formula one as well as McLaren." (GMM)
Click here to download Ron Dennis' original letter.