The Mail on Sunday said that video had emerged showing Mrs May praising Mark Clarke for his efforts on behalf of the party at an event in 2014.
She is said to have told him: “What you are doing is absolutely tremendous. RoadTrip 2015 is great. Thank you Mark for all you are doing,” before leading a round of applause by activists.
Clarke, nicknamed the “Tatler Tory” because the magazine once tipped him as a future minister, was suspended from the Tory party in 2015 and banned for life from re-joining after allegations a sex, blackmail and bullying scandal and the death of a young activist.
However the report said senior figures including the then co-chairmen Lord Feldman and Grant Shapps were not told of the allegations while Mr Clarke has always strenuously denied the claims against him.
The video of Mrs May, who was then home secretary, was said to have been shot during a by-election campaign in Rochester, Kent, in November 2014.
The Mail on Sunday also reported that Mrs May had since given a No 10 job to one of Mr Clarke’s “associates” – Jimmie McLoughlin, the son of the current Conservative Party chairman Sir Patrick McLoughlin.
There was no response from No 10 to the newspaper’s report. A Westminster source said: “Theresa is known as an active grassroots campaigner and would always support efforts to get Conservative MPs elected.”