Mohammad Ezam Mohd Noor has quit Umno over the party's apparent lack of commitment to tackle corruption.
Ezam, once regarded as former deputy prime minister Anwar Ibrahim's right hand man, had returned to Umno in 2008, after a falling out with PKR.
In a Facebook post, Ezam said he had returned to Umno during the time of then prime minister Abdullah Ahmad Badawi, who had committed to combating corruption.
He noted that Abdullah had kept his word by introducing the Malaysian Anti-Corruption Commission Act.
Ezam said two more issues that were close to his heart were the International Security Act (ISA) and Official Secrets Act (OSA) as he had been jailed under both laws before.
He noted that under Prime Minister Najib Abdul Razak, the ISA was abolished, but there was no progress on the OSA.
"I gave the benefit of doubt to the 'transformation' government of Prime Minister Najib Abdul Razak and did all I could to influence the administration make changes, particularly on the OSA.
"What happened, was the opposite. The OSA was not amended but, to be accurate, abused by making secret the auditor-general's report on 1MDB, which has nothing to do with national security that would warrant the use of OSA," he said.
"This was a million steps backwards in the transformation process professed by the government," he said.
Ezam said he had publicly criticised the government to correct this matter, but was instead demonised by certain leaders.
"Not a single Umno leader voiced out to rectify the situation.
"It was then I realised that I was no longer in a party that was serious about renewal, particularly when it came to issues of corruption.
"I now have made this tough decision to uphold my principles," he said.
Ezam said he had already resigned as communication director with the Finance Ministry six months ago, which was earlier than his contract expiry date, slated for April 1 next year.