It is understood that the office is resisting moves to join with Ashursts, arguing that the shift would remove any independence it has from its US parent.
Last week The Lawyer exclusively revealed that the two firms were engaged in talks.
But one source says: "Lathams in London is against it because it would damage its autonomy. Instead of running a nice little London office it would become part of Ashursts."
One industry insider says that the US office is committed to expanding its European practice through a merger rather than organic growth, and is limiting its search to those firms outside the magic circle.
One source says: "It has got some good people there who are very focused.
"I think that the firm likes the opportunity of being independent and of growing its own office.
"It wants to grow it in its own way, concentrating on its corporate, projects and banking department rather than becoming part of a larger entity."
Another source says: "There is not one US firm in London that is not thinking about a merger at the moment."
Joe Blum, managing partner at Lathams' London office, says: "We do not comment about speculation on mergers."