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The former rugby league star said on micro-blogging site Twitter he would remain fully committed to Northampton for the remainder of the campaign until switching clubs.
"I've decided to join Saracens from next season ... until then it's all about Saints," Ashton, 24, wrote.
Saracens confirmed the move in a brief statement on their website, saying Ashton had signed a long-term contract.
"We are obviously especially delighted but for now Chris remains a Northampton player. Out of respect for Saints, Saracens will not make any further comment on this matter," a Saints spokesman said.
Ashton made his debut at the end of England's 2010 Six Nations campaign and became established on the wing the following season, scoring six tries in the 2011 tournament to equal the all-time record for tries in a single championship.
