An NUJ spokesman said the documents showed Trinity Mirror plans to cull eight weekly Midlands titles. An announcement is expected within the next few days.
This would follow a swathe of cutbacks already at Trinity Mirror's regional operations. In March, it emerged that the company is closing four titles - the Black Country Mail Extra, Wolverhampton Adnews, Daventry Post and Ashby Herald.
The internal documents are reported to show Trinity Mirror is planning to turn the evening Birmingham Mail to an overnight paper and switch the morning business-focused compact Birmingham Post to a bi-weekly or weekly title in September.
Jeremy Dear, NUJ general secretary, claimed the cuts are being directed by the board of Trinity Mirror and "shareholders should step aside" and let others take over the titles if they are not prepared to provide "the resources necessary to ensure the people of the Midlands are properly served by their local papers".
He said Trinity Mirror needs to "come clean" about these plans and the community needs to "stand up for quality journalism".
A Trinity Mirror spokesman said it is constantly reviewing its business, particularly in the "current challenging economic environment" and, if it does have any plans to announce, staff "would be the first to know".
Newspaper entrepreneur Chris Bullivant has offered to save some of the weekly titles facing the axe.
Bullivant, who bought Observer Standard Newspapers from the administrator earlier this year, said: "I would love to take part in negotiations over the future of those titles in the Midlands that Trinity Mirror wants to close that aren't in conflict with our existing publications.
"We would undertake to save the jobs of as many staff as possible, given that the newspapers were viable. We would even consider allowing Trinity Mirror to keep the contracts for printing and distributing the titles - which would have considerable benefit for their workforce."
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