QBO, which sent in a pitch team bolstered by Good Relations and Bell Pottinger Corporate & Financial staffers, lost out to its consumer rival alongside The Red Consultancy and Shine Communications.
Lexis's annual fees are understood to be worth at least £500,000 per year as NU ploughs major investment into its marketing and brand comms for 2006.
The move follows NU owner Aviva's creation of its NU General Insurance business (NUGI) unit, set up to incorporate the insurance arm of the RAC, acquired eight months ago.
Lexis will work initially on briefs for NUGI, NU Healthcare and the RAC, including the British School of Motoring brand.
QBO is currently still working on projects for NU, as is Borkowski PR, which handles creative promotional work for NU home and motor insurance.