Mr Hughes, who was defeated by Mr Kennedy for the leadership, said Mr Kennedy had "never been a great policy promoter" or "an arden position taker". His outspoken remarks on the eve of Mr Kennedy's keynote speech to the conference will increase the belief among some in the Kennedy camp that Mr Hughes could become a problem.
Mr Hughes has the backing of many activists who share his suspicion of closer links with Labour. Mr Kennedy will give him the home affairs portfolio but his remarks will strengthen opposition to Mr Hughes becoming his deputy leader.
Alan Beith, the current deputy, is being urged to stay on for another year to keep Mr Hughes out of the post, which is elected by MPs.
"Paddy stuck his head out, went out front and everybody knew where he was going," said Mr Hughes. "With Charles, that isn't as clear and he will have to answer some of the questions."
Later, Mr Kennedy told the BBC's Newsnight: "Simon will be Simon, particularly during conference season ... One of the reasons he's so popular with the people within the party and outside the party is that he has a tendency from time to time to speak his mind."
But he added: "I think the members knew what they were doing when they decided who of the two of us should be leader."