CWA Canada union director Martin O'Hanlon said employees were notified of the plans on Wednesday.
"We've got no information that there's going to be any job losses at all, as a result of this," O'Hanlon said in an interview.
"You never know what could happen, especially in the press room because if you cut down one out of seven (editions) you never know what effect that could have."
Ingrid Bulmer, who represents workers at the paper, said staff were told the Sunday edition would be halted sometime in April.
A spokeswoman for the Chronicle Herald declined to confirm whether the Sunday edition would be discontinued.
"It's not an announcement we've made," said Jennifer Punch, senior manager of marketing and consumer sales.
"We do have a lot of exciting changes coming up in the future. We're in the planning stages of some changes that will have a positive impact on all of our platforms."
Further details will be revealed in the coming few weeks, she added.
The Chronicle Herald is the most widely circulated newspaper in the Atlantic provinces and the largest independently-owned newspaper company in Canada.
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