Stade Francais hooker Dimitri Szarzewski will be playing for cross-town rivals Racing-Metro as of next season.
It had been widely known for several months that the France international, a part of the team that finished second to New Zealand at last year's World Cup, would sign for Racing.
However, Szarzewski deliberately waited until after Racing's home match with Stade on Saturday at Stade de France, won 19-13 by his future club, before making his decision public.
"After seven years at Stade Francais I've made the difficult decision to join Racing Metro 92 ... to have a new experience and undertake new challenges," Szarzewski said.
"It's a difficult decision, but one I've reflected on for a long time."
The 29-year-old, who has 61 caps for France, signed for Stade in 2005 after arriving from Beziers and helped them win the Top 14 title in 2007.
Stade this season failed to make the top six for the end of season play-offs.
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