The videotape shows Mono Jojoy addressing his troops
President Rafael Correa of Ecuador has denied allegations that he received election funds from Marxist Farc rebels in neighbouring Colombia.
He said evidence had been fabricated to destabilise his left-wing government.
A video broadcast on Colombian television on Friday appeared to show a Farc leader saying he had contributed to Mr Correa's 2006 campaign.
The two countries broke off diplomatic ties last year after Colombian troops raided a rebel base in Ecuador.
The hour-long video was delivered to the offices of the Associated Press news agency by an unnamed Colombian government official and was later broadcast on national television.
AP said it was found on a computer seized in May in the Bogota home of a suspected Farc operative, and was decrypted last week.
It allegedly shows Jorge Briceno addressing his troops after the death last March of the Farc's founder Manuel Marulanda.
Mr Briceno speaks of money delivered to the 2006 election campaign of Ecuador's President Rafael Correa as well as meetings with his emissaries.
Mr Correa at first dismissed the video as a "sham".
"I personally don't know anyone in the Farc," he said.
The Ecuadorean president added that the release was part of a campaign by conservative groups "to destabilise the region's progressive governments".
"I can also go, now, and get a recording where the paramilitaries say that they financed the campaign of [Colombian President] Alvaro Uribe or of drug traffickers saying that they financed Alvaro Uribe's campaign," he added.
Analysts say that even if the videotape is genuine, it does not prove that Mr Correa personally had any knowledge of contacts with the rebels.
Relations between the two nations have worsened recently with an Ecuadorean court seeking the arrest of the former Colombian defence minister who had ordered the raid into Ecuador.
Ecuador is also imposing tariffs on Colombian exports.