He will leave at the end of September but will continue to work with the generic body in a consultancy capacity assisting on future European projects.
The organisation said its the Mendoza office would oversee the UK operation under the direction of Susana Balbo, president of Wines of Argentina, working alongside UK manager Andrew Maidment.
Commenting on its long-term strategy, it added that it would be assessing “opportunities Europe-wide”.
Forbes said: “I’d like to take this opportunity to thank the board of Wines of Argentina for giving me such a privileged and prestigious role in the world of wine. I feel that our work together over the past three years has put Argentina into its strongest position ever in the UK market and it has been a real pleasure to be involved with such fantastic people.
“Of course nothing could have been achieved without the dedicated and fulsome support of the member wineries, the amazing people behind them and the superb wines that they are making across the length and breadth of this vast country. Further the generosity and support of so many of you in the UK press and trade will be something I long remember.
“Lastly it almost goes without saying that it is my fervent belief that before long, in the eyes of the consumer and wine trade alike, Argentina will be universally recognised as the finest wine producer in the southern hemisphere.”
Balbo added: “James has been a great ambassador for Argentina and we thank him for his considerable efforts. Wines of Argentina will continue to have an extremely strong focus on the UK market and we look forward to building upon our current success to further raise awareness, promote and develop the demand for our wines.”