The office is resisting moves to join with Ashursts, arguing 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 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.
Joe Blum, managing partner at Lathams' London office, says: "We do not comment about speculation on mergers.