Barnett confirmed he will become the retailer's senior V-P for corporate affairs in Asia as of next January.
'My job will be to support the (management) team to achieve its business, financial and brand objectives,' he said.
Tesco Lotus, the Thai arm of the UK's largest supermarket group, declined to comment, but sources close to the firm say the appointment marks a move to take PR in-house.
Tesco Lotus has been a WS client for two years.
Former WS financial communications group director Peter Emblin will replace Barnett under the title SV-P.
Emblin cited building the financial PR practice as his main priority.