PPP-RTK is the integer ambiguity-resolved Precise Point Positioning (PPP). PPP- RTK extends the PPP concept by providing single-receiver users, along with the required satellite orbit and clock models, information about satellite phase and code biases, as well as estimates of the tropospheric and ionospheric delays. Such information reduces convergence times and improves positioning as compared to standard PPP. Uncertainty in the ionospheric signal delays is currently the main bottleneck for fast precise GNSS positioning. This is further exacerbated during periods of strong space weather conditions. During these periods there is an increase in the electron density in the ionosphere, further affecting the GNSS signal as it travels through the atmosphere. We present the effects of strong space weather conditions on the accuracy in ionospheric delay retrieval by the PPP-RTK network, and on the accuracy in positioning for PPP-RTK users. The results cover the periods 29 and 30 July 2016 (quiet conditions), and 05-08 September 2017, during one of the most significant space weather events in more than a decade.