Harrison is rumoured to be joining French Pro D2 side Provence, who currently sit eighth in their domestic standings.
And the move will bring the curtain down on a hugely successful stay in Northampton for the popular 29-year-old.
Harrison has spent a decade at cinch Stadium at Franklin’s Gardens, making 183 appearances and scoring 26 tries along the way.
The back row forward arrived at Saints in 2011 after being scouted by Dylan Hartley during a visit to his old school in Rotorua during the Rugby World Cup in New Zealand, initially joining the club’s Senior Academy before transitioning into the first team.
His breakthrough came during the 2015/16 campaign when he made 27 appearances for Saints, earned the first of five caps for England, and scooped the club’s breakthrough player, supporters’ player and players’ player of the season awards.
Harrison played a key role in Saints’ most recent trophy success, scoring a try in the final as the black, green and gold lifted the Premiership Rugby Cup in 2018/19.
That same season also saw Harrison score a memorable European hat-trick against ASM Clermont Auvergne, while the following campaign he took on the club co-captaincy alongside Alex Waller and notched up his 150th appearance in Saints colours.
But he will now exit for pastures new when his contract expires this summer.
Harrison said: “I feel humbled to have been a part of Northampton Saints for the last 10 years.
"I love playing for the club, and I love living in Northamptonshire, so this has not been an easy decision for me to make and I’m sure my family and I will come back one day.
“But I’ve been given the opportunity to experience a different league and style of rugby, and my family are looking forward to the new adventure ahead of us.
“Saints will always be a very special club to me. The players and staff here are like my brothers, and I want to thank all the supporters who have given me so many happy memories over the years.
"I’ll be giving everything I have to finish off this season with some silverware.”