Justin Timberlake is feelin' the heat amid Britney Spears' new book release -- and all the spilled secrets about their relationship -- but we're told his *NSYNC bros still have his back.

Sources familiar with the situation tell TMZ ... Lance Bass, Joey Fatone, Chris Kirkpatrick, and JC Chasez have nothing but love for Justin, and they're supporting him through all the harsh criticism due to "The Woman in Me."

We're told they've all known him for 25 years, and would be more than happy to be back performing with him ... despite all the negative energy Britney and her loyal fans are aiming at JT.

Remember, in her book Brit blames him for talking her into an abortion, accuses him of cheating and claims a run-in with Justin triggered a panic attack that ruined her 2007 VMA performance.

While *NSYNC fans and RCA Records would love another Reunion, it isn't in the cards right now -- we're told Justin's focused on his new solo single, album and a tour next year.

BTW, our sources say the hate JT's been getting isn't impacting his career plans. The single and tour for next summer are a full go -- however, we're told he's quietly removed some club appearances that were set for early next year, tied to the release of his single and album.

As we reported, there are no current talks about *NSYNC doing more together -- Justin said recording "Better Place" would just be a special thing for his 'Trolls' sequel.