Louis Picamoles has left Northampton to join Montpellier after just one season at Franklin’s Gardens for a record transfer fee worth at least £1m, the Guardian understands.

Picamoles signed a lucrative three year-deal last summer but his head has been turned by the prospect of returning to the club where he began his career. Northampton had previously stated they would not sell the France No8 but on Wednesday confirmed the deal, revealing they have received a “significant transfer fee” which is understood to be a seven-figure sum.

The 31-year-old was under increasing pressure to return to the Top 14 with the French federation (FFR) seeking to tie its leading internationals down to central contracts. Picamoles was the only member of the France squad to play South Africa this summer based overseas and his return can be considered a statement of intent by the FFR president Bernard Laporte, who is determined to improve the fortunes of the national team.

Northampton were determined to hang on to their prized asset, who made significant contributions for Saints throughout a difficult campaign and was named in the Premiership’s team of the season. Jim Mallinder, the director of rugby, has previously insisted that Northampton “were not a selling club” but once it became clear Picamoles wanted to leave, the club sought about ensuring the best possible deal financially. Northampton also insist the funds will go towards recruiting a replacement with Saints having scraped into next season’s Champions Cup last week.

In 1997 Newcastle signed Va’aiga Tuigamala from Wigan in a deal worth £1m but considering that fee included compensation and wages, Picamoles’s move effectively makes him the subject of rugby union’s first £1m transfer.

Picamoles has revealed that he was approached by Montpellier in an attempt to lure him back to France. They are bankrolled by the billionaire owner Mohed Altrad, who earlier this month claimed Picamoles had signed a “pre-contract” with the club and with the backing of the FFR, money was never an object.

“Montpellier approached me to see if I was interested in returning to France,” said Picamoles. “That coincided with the desire of the FFR to keep French internationals within the Top 14. I went to England for three years but I took the opportunity that was presented and difficult to refuse at 31 years old.

“I am proud to have played for Northampton Saints and I have enjoyed my time with the club. I am grateful for all the support and friendship I have received from the club, the coaches, my team-mates and supporters.”

Northampton’s Tom Wood has been banned for six weeks following his red card during the Champions Cup playoff final victory over Stade Francais last Friday. Wood was sent off for a stamp to the head of Djibril Camara and while he learned of his fate on Wednesday, he had already been dropped by Eddie Jones from England’s squad for the forthcoming tour of Argentina.