The publisher of the Sunday Mail and Daily Record is being urged to re-think its plans for journalist job cuts.
In a letter to Trinity Mirror, the Scottish Trades Union Congress said the closure of the News of the World had changed the newspaper market.
It said the Glasgow-based titles' owner had a chance to invest and boost sales.
Trinity Mirror is to print an extra 250,000 papers this weekend, in a bid to gain from the scandal-hit News International title's closure.
The company is also printing a further 1.5 million copies of the Sunday Mirror and the People, backed by a marketing push and cover price cuts in selected areas.
The Sunday Mail already leads the weekend market in Scotland, with average sales of 340,000 copies between last December and May.
The News of the World was biggest UK seller by far, with 2.67 million average sales recently, while its Scottish edition sold 245,000.
Trinity Mirror announced last month that it intends to cut 90 jobs from the 240 journalist staff at the Sunday Mail and its Daily Record stablemate.
Grahame Smith, general secretary of the STUC, has written to Mark Hollinshead, managing director of Trinity Mirror in Glasgow, to ask that plans to cut jobs should be reassessed in light of News International's difficulties.
Mr Smith said: "It makes no sense to continue with the planned job cuts when a huge opportunity has now presented itself for Trinity Mirror to re-establish the Record and Mail as Scotland's premier popular newspapers.
"With its major competitor in such serious difficulty, now is a time for investment at Trinity Mirror in Scotland, not cuts."
A spokesman for Trinity Mirror declined to comment on the letter.
With the closure of the News of the World in Scotland, 21 journalists were given redundancy notices, and 38 freelance journalists are also affected.