"This was easily the hardest decision I have had to make (and I've made a few)," he wrote. "It will be a real wrench to leave.
"I feel the experience and opportunity to play in France was too good to turn down. My focus remains on giving my all to Sarries for the coming season."
The 29-year-old will have spent five years with Saracens during which he became the club's leading try-scorer and won two Premiership titles as well as a European Champions Cup.
But he is currently serving a 13-week ban for biting, his second suspension of 2016, and has not featured in the England team this year.
Ashton had said he was hopeful of winning back his place under coach Eddie Jones, but the move to France means he will be ineligible under the rule preventing players from outside the Premiership being selected.
Before Ashton's move to Toulon was confirmed, former England fly-half Paul Grayson told the BBC he believed his international exclusion would influence Ashton's decision. "As a player you just think 'I am up against it here, I am not going to be playing for England, where is the new adventure?'," he said.
Ashton, who scored 19 tries in 39 tests, became a firm favourite with the England fans after switching from rugby league, where he played for Wigan, to join union side Northampton in 2007.
Ashton make his international debut in 2010, marking each try with a trademark swallow dive that soon became a highlight for England supporters. He established himself as a firm fixture on the wing until losing his England place before the 2014 Six Nations.
Although he was included in the training squad for last year's World Cup, he did not make Stuart Lancaster's final squad. Jones, Lancaster's successor, included him in his squad for the 2016 Six Nations, but Ashton missed the tournament after being banned for 10 weeks for eye-gouging.
Throughout his disciplinary troubles, Saracens stayed loyal to a player who has been crucial to their success since joining at the start of the 2012-13 season, thanking him in a statement issued by director of rugby Mark McCall on Tuesday.
"We would like to thank Chris for his contribution to Saracens over the last five seasons and the role he played in helping us create some incredible memories during that period. Chris will still have an important role to play this season and we’ll look forward to seeing him back in action after his suspension."
(Reporting by Neil Robinson; Editing by Pritha Sarkar and Andrew Heavens)
