Jennifer Lopez will not be hitting the road this summer after all -- as the singer is canceling her planned tour outright, even before it starts ... TMZ has learned.

A rep for Live Nation tells us Jen's "THIS IS ME...LIVE" tour -- which was supposed to get underway in June and hit a bunch of North American venues through August -- is getting completely scrapped.

The reason ... we're told, J Lo is "taking time off to be with her children, family and close friends." Jen herself also addressed the cancelation, hopping on her site OntheJLo to say, ... "I am completely heartsick and devastated about letting you down. Please know that I wouldn’t do this if I didn’t feel that it was absolutely necessary."

She adds, "I promise I will make it up to you and we will all be together again. I love you all so much. Until next time…"

No other official explanation was provided, but we have a bit more insight. While there were rumblings of weak ticket sales earlier this month, sources with direct knowledge tell TMZ this was NOT a factor in the decision ... as the tour was even seeing an uptick in ticket sales in key cities.

We're told this was a very difficult decision for J Lo and, -- and like she expressed in her note, she's bummed. Of course, there's other stuff going on in Jen's life right now that could possibly be pertinent to this ... which seems to be reflected in the official reasoning.

Jennifer and her husband, Ben Affleck, have faced public scrutiny over their marriage. The couple has sent several mixed messages about the state of their relationship ... stepping out a number of times together, despite currently living apart.

LN says fans who purchased tickets through Ticketmaster will be automatically refunded. However, if you bought a ticket through a 3rd party reseller -- SeatGeek, StubHub, VividSeats, etc. -- you're encouraged to reach out to them directly for more details.

Remember, Ben started staying at a Brentwood home earlier this spring ... with Jennifer residing in their Beverly Hills abode -- which they scooped up for $60 million just last year.

Neither Ben nor Jen have addressed the situation publicly ... but this tour nixing is either good news or bad news for them -- hard to interpret it, really. Either way, no Jen on tour for now.