OTTAWA - Stephen Harper's troops aren't laughing at an apparent fat joke directed at the prime minister by his chief rival Stephane Dion.

Liberal leader Dion made a reference to the prime ministerial paunch this week during a TV interview in which he suggested Harper might need some exercise.

Expressing outrage at the wisecrack, senior government officials distributed transcripts of the joke to reporters Wednesday and offered up spokespeople for official comment.

One Tory cabinet minister said the joke was unbecoming of a man who aspires to be prime minister.

"We're partisan on a regular basis. But as a fellow politician right now I'm really embarrassed,'' said Helena Guergis, junior minister for sports and foreign affairs.

"Essentially what he's done is called the prime minister fat.''

She said "real leaders ... do not go around making fun of other people's weight.''

That dig at the Liberal leader echoes new TV ads in which the Tories question Dion's leadership qualifications.

It was a discussion about those ads that led to Dion's joke in a Global TV interview. He said that negative ads were not his style and that he wouldn't stoop as low as the Tories had.

The Global reporter then pointed out to viewers that Dion wasn't above taking personal shots of his own.

The Liberal leader joked that the exercise room at Stornaway -- the official residence of the leader of the Opposition -- had been refurbished.

And he said the new workout room would benefit his successor.

"Maybe Mr. Harper will have the opportunity to lose his (extra) weight now because we have this exercise room,'' Dion said.

Guergis said she learned about the joke when her mother called from Angus, Ont., and sent an email to say she'd seen the TV exchange and was appalled by it.