John Stamos got really close to Britney Spears ... or at least a "Glee" cast member who looks a lot like her!

Stamos just Twittered this shot from the set of the show, showing him being straddled by Heather Morris in Brit's famous nude "Toxic" bodysuit, along with the message ""not the real @, but an incredible simulation."

The pic is just another glimpse of tonight's "Britney/Brittany" episode -- where Stamos makes his debut and Ms. Spears herself will make a cameo.

Take a closer look at the pic and more shots from the episode in the gallery above!

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