The blame is being pinned squarely on Honda, who in three years have failed to produce a competitive engine for McLaren.
"McLaren are understood to be speaking to Mercedes about going back to using their engines," Ben Hunt, the correspondent for the Sun newspaper, said.
Another report, in Germany's Auto Bild, said McLaren shareholder Mansour Ojjeh has reached out personally to his old contacts at Daimler.
But Spain's El Confidencial quotes a source who claims Mercedes is not McLaren's only option, as "phones are running hot" in F1 circles at present.
However, McLaren's situation is complicated by the huge sponsorship deal that comes with the Honda partnership.
Former McLaren driver Martin Brundle is quoted by the Daily Mail: "They can't leave Honda unless Honda pull out and leave them a fat cheque like they did Brawn.
"They are very much integrated technically, financially and they have to work through it," he added.
Indeed, new McLaren executive Zak Brown insisted the Woking team "will honour our deal, as McLaren always does".
The media reports, however, insist that the initial contact between McLaren and Mercedes about an engine supply really did take place.
"What does the improvement that we need depend on?" El Confidencial quotes Fernando Alonso as having said last week. "It's going to depend on the big decisions that are taken at the highest level."
Mercedes would not comment, but a McLaren spokesman said: "Winter testing was challenging and disappointing.
"We are working with Honda to address shortcomings and deficiencies. Together with Honda we are considering options, but we will not comment on media speculation."