Declan Donnelly told The Times he finds it difficult to picture Miliband as prime minister, while Ant McPartlin said he no longer knows which philosophy Labour stands for. In a further comment, which did not make it into the newspaper, McPartlin said he is "a real floating voter".
But Miliband seems confident that he can win back the support of the pair. Speaking at an event in Lincoln, Miliband said: "I think they said they were floating voters so I've got 78 days to persuade Ant and Dec back on board, and I'm sure we can.
"Look, I think what I would say to Ant and Dec, and the whole country, is I think there's a big choice at this election.
"How do you want the country run? Do you want a government for the rich and powerful or do you want a government for everyday people. That's the government I'd lead."
Ant and Dec, winners of 27 National Television Awards for popularity and famous for shows stretching from Byker Grove in the 1990s to I'm A Celebrity... Get Me Out Of Here! and Britain's Got Talent now - describe themselves as "staunchly Labour" but said they do not know how they will vote on May 7.
Explaining his doubts about Miliband, Donnelly told the Times: "I'm not sure I could picture him as prime minister."
And McPartlin said: "I voted Labour all my life. But last election I voted Tory because I was thoroughly disillusioned with the Labour government.
"Now I'm thoroughly disillusioned with a coalition Government. I feel we're both staunchly Labour and would vote Labour if we could, but I don't know what their philosophy is any more."
Writing on Twitter, interviewer Helen Rumbelow revealed: "Not in interview but Ant told me 'I'm looking forward to seeing their manifestos. I'm a real floating voter now'."