The former No.10 spin doctor risked infringing party rules with the admission, which came on Sunday evening as results in the EU-wide elections came in.
Campbell, who worked under Tony Blair and has campaigned for a second Brexit referendum, said: “I voted Liberal Democrat. I didn’t vote Labour for the first time in my life.
“It was a very very strange feeling but I think on this issue, at this time, the Labour Party has let its own supporters down, its let its own members down, and its let the country down in the way it has failed properly to devise a policy that the country could unite around and the way it failed to campaign.”
Voting for a political party other than Labour where a Labour candidate is standing can risk immediate expulsion, according to party rules.
Campbell admitted last week that he had voted “for a Remain party” but had previously refused to elaborate.