*NSYNC is back in the zeitgeist thanks to ‘Deadpool’ — and while there’s renewed pressure for a reunion, it won’t happen unless they’re all on board … so says Chris Kirkpatrick.

Here’s the deal … the ’90s boy band’s hit song ‘Bye Bye Bye’ blasted into the Top 20 on Spotify’s global chart this past week more than two decades after its original release — all because Ryan Reynolds featured the tune in the opening credits of ‘Deadpool & Wolverine.’

Since their names are being thrown around again as a result of the movie — folks are yet again clamoring for the guys to get together for a tour (or even new music) … but Chris tells TMZ there’s a major hang-up that might prevent that from ever happening.

For starters, there’s already been interest in *NSYNC getting together again — dating back to the ‘Trolls 3’ movie — and CK confirms ‘Deadpool’ is only adding more fuel to that fan fire.

Amid all that fervor … Chris says there’s been rumors — or perhaps just an assumption — that he, Lance Bass, JC Chasez and Joy Fatone would do an *NSYNC reunion without Justin Timberlake, who hasn’t shown any interest in this, but Chris shoots that notion down.

As he explains to us here … an extended would-be *NSYNC reunion of any kind is all or nothin’ when it comes to its members — and Chris says they ain’t doing jack squat without JT.

His rationale basically boils down to the fact that they’re a group — and breaking up the group just isn’t going to happen … even if he and the other fellas are perhaps more eager to get something going than Justin is. He makes it clear … no Justin, no *NSYNC.

We know Justin is still friendly with the guys — he did recruit them for a new song on his most recent flick, after all — but he hasn’t dropped any explicit hints that he’s interested in getting the band together officially. Of course, he’s on his own world tour right now.

Anyway … in terms of how ‘BBB’ came to be used in ‘Deadpool’ — Chris gave us the backstory on that too … and it’s pretty neat. He says Ryan approached them and gave them a sense of how it’d be incorporated … but when they saw the final product, they were wow’d.

Of course, because the movie’s a smash hit right now … that means ‘Bye Bye Bye’ is back on the charts — but like Chris says in no uncertain terms — fans can probably cool it on calling for a reunion … at least until JT gets on board, and it’s unclear if he will.