The union described Trinity Mirror's acquisition of 32 of GMG's regional newspapers as "a very serious development" as it will lead to the UK's largest regional newspaper publisher assuming control of MEN Media and its flagship Manchester Evening News title.
Trinity Mirror's £44.8 million acquisition of GMG's regional media business means it will become the dominant media group in north-west England. It already controls two large daily newspapers in Liverpool and 16 other titles across the region.
The NUJ called for the new owner to make a series of guarantees about protecting the quality and content of the 22 newspapers that it will now control in and around Greater Manchester.
Michelle Stanistreet, NUJ deputy general secretary, said: "The move by Trinity Mirror has implications for the future ownership of local media across the UK, as major publishers weigh their chances of swallowing up what remains of independent publishing in Britain.
"The concerns for diversity of information and media in the North West are an immediate issue which we will be raising with the Department of Business Innovation and Skills.
"Trinity Mirror already publishes the Liverpool Post and Echo titles, in addition to a substantial stable of weekly papers across Cheshire and Merseyside.
"Now it will also control the prestigious Manchester Evening News, which has a series of titles in the region. This all raises issues of editorial independence which should be of concern to government and community leaders as well as the media industry.
"The NUJ's immediate priority is to secure the employment of the MEN workforce and to have real guarantees from Trinity Mirror that the quality and content of MEN publications will be protected.
"We are determined that the jobs and conditions of current employees will be fully protected, and we will be seeking urgent discussions with management to establish investment plans for the development of the papers."
Oliver Luft writes for Press Gazette