© Rex Features / Stephen Butler
The presenter admitted the mistake while hosting Channel 4's Comedy Gala at London's O2 arena earlier this week, The Sun reports.
Recalling a visit to Great Ormond Street Hospital, Ross recalled how one child had asked him to "do your song".
"I told her, 'Why don't you start my song and I'll join in?'," he told the audience. "So she said, 'All right' and sang, 'Go Compare! Go Compare!' She thought I was that fat tw*t who sells insurance.
He added: "I wish I could say I was joking!"
Evans, who plays Gio Compario in the advertisements, was also at the fundraising event.
"I laughed my head off," he said afterwards. "I can honestly say that I've never been mistaken for Jonathan Ross."
Ross is currently filming a full series of his magic show Penn & Teller: Fool Us for ITV1.
> Jonathan Ross 'planning radio comeback'
