A majority of respondents, 53%, believe the allegations, despite his denials. Among Republicans, 39% believed them to be true.

"The most striking thing is that Herman Cain is actually seeing a fairly substantial decline in favourability ratings towards him, particularly among Republicans" said Chris Jackson from Ipsos.

The Republican presidential candidate, who had been enjoying a surge in the polls which made him the latest challenger to frontrunner Mitt Romney, is struggling to draw a line under allegations that he sexually harassed at least three female employees when head of the association. Two women reportedly received financial settlements after the incidents.