He said in an interview with Нromadske.ua, Censor.NET reports.
According to him, the materials include evidence of parliamentarian graft, asset-grabbing and corrupt talks with businessmen.
"It's about voting, asset-grabbing, talks with certain businessmen on a number of issues, on how much money they owe him. These are considerable sums, about $300 million. He has criminal cases opened against almost every lawmaker, he keeps them hooked. This is his manner of communication in politics," he said.
According to Onyshchenko, he did his best to record Ukrainian president's corrupt practices and negotiations with his representatives.
"Knowing Poroshenko as a man who cheats everyone, I decided to play it safe. So when they came to London for negotiations, I got in touch with the British intelligence who recorded everything," he said.
The MP says the facts were collected during two years of his being part of Poroshenko's team. He does not deny his responsibility for involvement in the corrupt schemes of the president.
"I've worked with the president during last two years, carried out all his instructions, I don't deny it," Onyshchenko said.
Earlier today, Onyshchenko, who is staying out of Ukraine, said he had handed compromising materials on President Poroshenko to the U.S. intelligence.
On March 16, the NABU raided the office of Onyshchenko. It stated the searches were carried out under the pre-trial investigation into abuse of office during the sale of natural gas extracted jointly with Ukrhazvydobuvannia SE.
Later, the NABU requested the Prosecutor General's Office (PGO) to lift the immunity of people's deputy Onyshchenko. Moreover, the NABU and the PGO claim Onyshchenko is involved in embezzlement of 1.6 billion hryvnia (approx. $64.02 million).
On June 16, Prosecutor General Yurii Lutsenko issued a motion requesting Verkhovna Rada's consent for criminal prosecution, detention, and pretrial confinement of Onyshchenko. On July 5, the parliament approved the detention and arrest of the people's deputy.
The suspect himself stated he was busy with the preparation for the upcoming Olympics and not going to react to NABU's actions. On July 29, Onyshchenko said he was in the U.K. and had applied for political asylum.
On Dec. 1, 2016, the Security Service of Ukraine said it suspected fugitive MP Onyshchenko of the high treason. The MP reportedly acquired Russian citizenship in exchange for cooperation with Russian authorities to destabilize the situation in Ukraine.
Related materials: Onyshchenko case