Prentice was responding to Wildrose leader and former MP Brian Jean, who said the incumbent Alberta premier wanted the prime minister's job.
Jean told CTV Edmonton's morning news show Thursday that members of Prentice's team would try to undermine Harper whenever the prime minister's polling numbers would sink.
"It did happen," the Opposition party leader told the host.
"Polls go up and down, and some people would try to actively undermine leaders, and they have done that before. And certainly when polls go down, we found that some people would actively try to undermine people that were in leadership. And sometimes the people that were on his team would do that."
Prentice scoffed at the notion.
"I think it is ridiculous," he said. "The prime minister and I are quite close and had a fabulous working relationship all the time I was in Ottawa."
Jean later offered an apology.
"It was inappropriate of me to talk about past internal caucus tensions," he said in a statement.
"I owe my former colleagues an apology. I won't be speaking further on this topic — it is a distraction from the important issues facing Albertans in this election."
Voters in Alberta head to the polls May 5.
Jean and Prentice served together in the federal Conservative government under Harper until Prentice retired from federal politics in 2010.
Prentice held several cabinet posts while Jean served as a parliamentary secretary to the minister of Transport.
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