Witherspoon was in London last week (02Dec09) to speak to Members of Parliament about domestic violence issues as part of her role as honorary chairwoman of the Avon Foundation.
The British leader later stood up to thank Witherspoon for her efforts, and accidentally mixed her up with Zellweger.
But the Walk the Line star has laughed off the incident, admitting it's happened frequently in her career.
She says, "He mixed me up with Renee Zellweger. That's OK - she and I get mixed up a lot. He said something in Parliament about how he had heard me speak at (filmmaker) Anthony Minghella's funeral but that wasn't me, it was Renee. She's lovely and she's short and blond and southern. So I understand!
"She actually called up a (mutual) friend of ours and said she'd heard about it (Brown's mistake) - it happens all the time. I have a dog and I took the dog to the vet, and they said, 'No no, we're waiting for Renee Zellweger's dog' and I said, 'Actually this is my dog.' Everybody gets us confused a lot. But there's worse people to be confused with other than a lovely actress!"