Cuckoo will see its full fibre availability expand nationally to an initial 17m plus homes through its ability to access AllPoints Fibre Networks (APFN) Wholesale platform.
CityFibre's full fibre network is now available to over one million UK homes via more than 30 consumer ISPs.
The milestone puts CityFibre on track with its £4bn investment programme to reach eight million homes by 2025. By the end of 2022, it plans to have builds underway across more than 150 cities, towns and villages before expanding to 285 by 2025.
CEO Greg Mesch said: "We are focused on supporting our consumer ISP partners to drive service take-up. We look forward to collaborating with our partners in the coming months to design new packages and initiatives, and to continue raising awareness of the benefits of full fibre."