A media release sent on behalf of Courtney says the 2010 Supercars champion has elected to leave the Clayton squad due to a yet-to-be-disclosed opportunity that was presented to him for the 2020 Virgin Australia Supercars Championship and beyond.
It is understood Courtney will reveal his 2020 plans in the coming weeks.
“My decision not to finalise negotiations on a new deal with Walkinshaw Andretti United is one that I've not taken lightly given the close personal friendships I've developed within the team, but the time has come to take on a new challenge,” said Courtney.
“I love this team. WAU's been my home for nine years, and we've been through a lot together, but an exciting chance has presented itself, and it just couldn't be ignored.”
The out of contract Courtney has been a long term fixture at the Walkinshaw outfit having joined the team for the 2005 enduros, then on a full-time basis from 2011 after winning the Supercars title at Dick Johnson Racing.
Since then, Courtney has won seven races, including back-to-back Clipsal 500 titles in 2014 and 2015, and visited the podium a further 27 times.
However, his future at the team for 2020 was uncertain with Chaz Mostert widely expected to join the Holden operation for 2020.
Courtney's announcement has now thrust the spotlight on the team's future plans with its other current driver Scott Pye also out of contract.
Speedcafe.com understands Pye has held positive talks with WAU for next year but has also been linked to rival teams including Tickford Racing.
It means WAU may field a completely changed line-up for next season.
WAU has since confirmed Courtney's departure after nine seasons at the team.
Team co-owner Ryan Walkinshaw has acknowledged his driver's tenure at the team, admitting that he will be ‘missed' moving forward, before teasing that a driver announcement will be made ‘very soon'.
“James has been a great member of this team for nine seasons, and we've shared some fantastic times together,” said Walkinshaw.
“I'm sure we will all miss his vibrant personality and smile around the garage, but we are looking forward to finishing our time together on a high for the remainder of the 2019 season.
“We all wish him the best for the future, he will always be a friend of the team and my family.
“We look forward to announcing a new WAU driver to our fans very soon.”