Copé quit following claims that invoices for former president Sarkozy's campaign were fraudulently billed as party expenses.
Speaking on TV on Tuesday night, Copé asked France not to doubt his integrity, adding that employees had "abused" his trust.
Copé, who will step down on June 15th, said he would “turn the page” after the scandal, dubbed the Bygmalion affair after the name of the PR firm that was authorizing the phony invoices.
“I will now handle politics differently, but always with the same idea that has always driven me, which is my love for my country and serving the French people,” Copé told TV viewers.
“I ask the French not to doubt my integrity. My honesty is total,” said Copé before blaming his colleagues in the party.
“I discovered all this 12 days ago, when it was published in Liberation [newspaper]. I trusted those whose job it was. Employees have abused my confidence.
“As president of the UMP I of course have this overall responsibility. But there was no warning system, so I only got wind of it 12 days ago. When I heard about it, yes, it was like the sky falling down on me.”
The shocking revelations of the scandal emerged this week when Sarkozy's former deputy campaign director gave an explosive television interview on Monday evening.
Jérôme Lavrilleux tearfully claimed that bills for Sarkozy's failed 2012 re-election campaign were passed off as invoices for party meetings in order to skirt round France's strict limits on campaign financing.
Lavrilleux claimed that neither Sarkozy nor Copé were aware of what was being done. "I didn't have the courage to say 'stop, we have gone too far'," he said.
According to a lawyer for the company, the amount billed to the UMP that should have been charged to Sarkozy's campaign amounted to more than €10 million ($13.6 million).
The lawyer says the company was pressured to falsify the bills or risk not getting paid.
While Bygmalion has sought to explain the false bills as an attempt to help Sarkozy, some senior UMP figures suspect the company of simply seeking cash, with the connivance of Copé.
The saga is the latest in a series of scandals linking Sarkozy to alleged irregularities in the funding of his electoral campaigns which threaten to wreck his chances of reclaiming the presidency in 2017.
The drip of corruption allegations against the UMP has been credited with helping to fuel a surge in support for the far-right National Front (FN), which topped the polls in Sunday's European elections with an unprecedented 25 percent share of the vote.
FN leader Marine Le Pen immediately went for the jugular saying the scandal "created a very serious situation for democracy in France" and adding: "I cannot see how Nicolas Sarkozy can escape his moral responsibility."
Current French President François Hollande warned that with such scandals "it's democracy that could be affected, even infected," warning of the political capital being made by the far right by the UMP's troubles.