Hornby, who hasn’t played a first team game since the league two promotion play-off final at Wembley in May, 2013, was told the news today by manager Chris Wilder.
The 19-year-old’s Sixfields career is now over and he is free to try and find a new club.
“The boy hasn’t played football for two years, and sometimes you come to a decision between yourself and the coaching staff that maybe he needs to get himself up and running somewhere else,” said Wilder.
“He maybe needs a new environment to get himself working at.
“It is a shame because when I came to the club people were talking about him in good light, but I haven’t been able to see him kick a ball really, not even in training for quite a while.
“But hopefully he will get over his injury problems and get himself going.”
Hornby, who is Northampton-born and came through the youth team ranks at Sixfields, broke into the Cobblers first team under then manager Aidy Boothroyd as a 17-year-old in the 2012-13 season.
He made 29 appearances in all as Town reached the play-off final, where Hornby was a second-half substitute as Bradford City romped to a 3-0 victory.
Hornby, who signed a two-and-a-half year Sixfields deal in January 2013, developed a back problem the following summer and although he did play in pre-season before the injury was properly diagnosed, he was missing for the entire 2013-14 campaign.
He did manage to overcome that long-term problem, and has been fit enough to make the bench for the first team on various occasions this season, most recently at AFC Wimbledon on Good Friday, but has not taken to the field.
A series of knee and groin problems mean he hasn’t made an appearance for Wilder.
“When I spoke to Lewis he understood, and maybe I got the vibe from him as well that a change of environment might be best,” said the Cobblers boss.
“He has been here for quite a long while, and it might be best for both parties.
“It’s a situation for us going forward that, not just with Lewis but with others as well, we have to make sure our recruitment is right in terms of the players that we bring in are reliable and robust.
“I’m not saying we won’t ever take a chance on a player, but 90 per cent of the players we bring in we have to make sure they have played their games, and they are going to be able to cope with the rigours and demands of a tough league two campaign.”