Guardian Media Group, the publisher of The Guardian and Observer, is considering the radical changes as part of its plan to cut costs by 20% and break even within three years, according to Reuters.
The newspapers have been published in Berliner-style format since 2005, moving from the broadsheet format, and was introduced by former editor Alan Rusbridger.
A Guardian News & Media spokesperson said: "We don't comment on rumour or speculation."
A possible deal with Rupert Murdoch’s News UK would follow an industry-wide initiative to pool newspaper ad sales. That plan, known as Project Rio, was dealt a major blow last week after the Daily Mail pulled out.
Campaign broke the news that publishers were discussing joint ad sales last May as print ad revenues slumped by nearly 20% and advertisers shifted money into Facebook and Google.