A new free daily newspaper launched in Cardiff on Tuesday could undermine existing papers which have already seen their readership drop, it is claimed.
Metro, which is available in 14 other cities, contains UK and overseas news aimed at "young, time-starved" readers.
Media expert Prof Kevin Williams said the launch could "undermine the viability of the existing papers."
The company has played down any threat, saying the paper will complement and boost existing business.
When Metro was first published in London in March 1999 as a free colour morning paper for commuters, circulation of the London Evening Standard dropped by 12% and continued to fall.
According to figures from industry analyst ABC and the Newspaper Society, the South Wales Echo was selling nearly 90,000 copies a day in 1989.
Today it sells around 55,000 copies and the Western Mail has fallen from 75,000 in 1989 to just over 40,000.
But Prof Williams, lecturer in media studies at Swansea University, said it was about more than just sales..
"They can't survive at existing circulation levels," he said.
"Anything that competes for advertising revenue is going to be a real threat and could be disastrous for the Western Mail and Echo."
'Affluent and urbanite'
But the Trinity Mirror Group - which is printing the Metro - said it would boost its business, complementing existing titles and targeting "a non-traditional newspaper reader, who is young, affluent and urbanite".
Metro does not have specifically Welsh news, but contains what it calls "no-spin national and international news". Its south Wales content will be limited to entertainment information.
The company said it was a model that had already proved successful in Birmingham, Newcastle and Glasgow.
Western Mail and Echo managing director Keith Dye said the publication was meant to be a "quick digest of what's happening around the UK and the world" and was not in direct competition with the organisation's Cardiff-based titles.
He said: "The Metro is aiming to be a quick, 20-minute read for people commuting to their office on their way to work and, if you want to have in-depth news about what is happening in Wales or in your local area, you'll still go to your local paper for that.
'Buying habit'
"It's meant to be a national newspaper, it's not trying to compete with the Western Mail or the Echo or any of our other titles. It's meant to provide a broad, quick read."
He said the initial print run of around 25,000 copies, available between 0600 - 1000 GMT at selected train stations, on buses and in the city centre, would be reviewed later.
"What the Metro does is it reaches a new audience, it's trying to catch younger professional readers, people who are not naturally into newspaper readership.
"So if it gets them into the buying habit then, long-term, they've got a better prospect of buying one of our other titles."
Metro said its UK circulation of 1.1m copies made it the fourth biggest weekday newspaper in the UK.
