Party leader Nick Clegg, a pal of the actress' director-husband Sam Mendes, was left guarding his pride after a snap of the Oscar winner was made to appear promoting a Lib Dem awards ceremony.
The picture, apparently showing her at the recent Lib Dem Council Awards in Harrogate, was featured in the party's conference brochure.
It allegedly offered the belief that Winslet, who has always sealed her political affiliations, was a Lib Dem supporter.
"They did not request permission to use the image and, so far, Kate has not been made aware of its use. I am not aware of her political position," the Independent quoted her rep as saying.
However, a Lib Dem spokesman insisted: "It was a humorous piece for a fairly low-key event."
On being questioned if the ad backed the idea of depicting the star as a party supporter, he added: "That is not the idea.... We are more than happy to make it clear that it is not the case." (ANI)