A spokesman for Saracens added: "We are obviously especially delighted but, for now, Chris remains a Northampton player.
CHRIS ASHTON FACTFILE
Born: Wigan, 29 March 1987
Scored 25 tries in 45 games for league side Wigan
Switched to union in 2007 to join Northampton Saints
Saints: 95 tries in 105 games
England debut: 12-10 Six Nations loss to France in March 2010
England: 15 tries in 18 Tests
"Out of respect for Saints, Saracens will not make any further comment on the matter."
BBC Look East understands that Saints director of rugby Jim Mallinder will not include Ashton in his starting line-up for Saturday's Heineken Cup Pool One visit of Munster.
The move brings to an end months of speculation over the future of Ashton, who has been allowed to speak to other clubs since New Year's Day.
Saints had said they would not
break the bank to keep him at the club
, but it is understood that the player's eagerness to leave was motivated more by a desire for a change of scenery after four years at Franklin's Gardens.
Reacting to Ashton's departure, Mallinder said: "We're obviously disappointed that Chris has decided he wants to play his rugby elsewhere.
Ashton runs in four England tries
"That said, we have known for some time that Chris was looking elsewhere and had been planning to use that part of the salary cap by strengthening our squad in a number of positions.
"We've also got several good wingers at the club who have all been pushing for a first team spot this season, the likes of Jamie Elliott, Vasily Artemyev, Scott Armstrong, Paul Diggin and Noah Cato, plus Anders Mogensen in the academy who has been playing well on loan for Cambridge."
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