Our sources tell us Britney is not in mental or physical shape to resume rehearsals for the series of shows, which were supposed to kick off back in February.

Britney's longtime manager, Larry Rudolph, tells TMZ, "As the person who guides her career -- based on the information I and all of the professionals who work with her are being told on a need-to-know basis -- from what I have gathered it's clear to me she should not be going back to do this Vegas residency, not in the near future and possibly never again."

Rudolph is not involved in Britney's conservatorship -- never has been -- and he is only given medical information that directly impacts Britney's career. He says Britney was 2/3 of the way through rehearsals for the reboot when her dad got ill late last year.

As Rudolph put it, "It was the perfect storm. We had to pull her show because her meds stopped working and she was distraught over her dad's illness." The residency, which was supposed to kick off February 13, was put on ice.

Rudolph tells us it's always been up to Britney when she wanted to work and when she didn't. He said, "Last summer, when she wanted to tour, she called me every day. She was excited. She hasn't called me in months. Crickets. She clearly doesn't want to perform now."

As for the future, Rudolph says, "I don't want her to work again 'till she's ready, physically, mentally and passionately. If that time never comes again it will never come again. I have no desire or ability to make her work again. I am only here for her when she wants to work. And, if she ever does want to work again, I'm here to tell her if it's a good idea or a bad idea."