Rumored but never verified. I personally think she doesn't have them. They aren't THAT big and they look pretty natural.
The main reason was how different she looked in the Sometimes video compared to BOMT. What people failed to notice is that she was much thinner in the BOMT video and also breasts are usually still growing at age 17. A lot can change in 6 months when you are a teenager. Plus she could have started to use a push-up bra to enhance her growing boobs
The easiest way to find out if a woman has implants or not is to look at her armpits to see if there is a scar. You can clearly see it on Christina (which has confirmed she has implants) while there is no sign of it on Britney.
Anyway I don't think either Shakira or Britney look plastic. But Shakira's face has much less personality than Britney's. It's about more than looking just looking young and fit.
Also if you think Britney is out of shape then you really have anorexic mindset. She has more flesh than many of her peers in the business but I think that's rather a positive than negative thing and far from being overweight.