The transfer comes as part of the next phase of the integration of the two firms after they completed a £31 billion merger in 2021.
Virgin Media O2 said the switch will happen seamlessly for the vast majority of customers and will not require people to replace their SIM cards or change any billing information.
The company added that customers will not see the cost of their plan rise as a result of transfer and millions will receive unlimited texts and voice calls, as well as an increase in their data allowance.
The switch will roll out in stages throughout 2023, with the first group of customers to begin receiving letters about the change in February. The migration will then get underway from March.
Customers affected by the switch will be told exact details of the change, including full details of their new plan and when they will move over, at least 30 days before their migration takes place.
The company said it aims to have all existing and newly joined Virgin Mobile customers on O2 plans by the end of the year.
Virgin Media O2 chief commercial officer Gareth Turpin said: “This is a major milestone moment for Virgin Media O2 as our Virgin Mobile customers start moving over to O2 plans, receiving added value and benefits on top.
“Our teams will guide customers through every step of the migration, and we’re laser-focused on making sure this all occurs in the most hassle-free way possible.
“With all of our mobile brands now powered by the award-winning O2 network, we are making fantastic progress in our integration plans while continuing to deliver a range of knockout mobile services that cater for all needs.”
The Competition and Markets Authority (CMA) gave the merger the green light in 2021 after an in-depth investigation.
When the deal was announced, the companies said it would create a “full converged platform” for customers, and will mean an investment of £10 billion in the UK over the next five years.
A Phase 2 investigation found that the deal was unlikely to lead to any substantial lessening of competition, with investigators concluding that the costs of leased-lines are only a small element of rival mobile companies’ overall costs. As such, it is unlikely that Virgin would be able to raise leased-line costs that would result in higher charges for customers.
Investigators added that competitors in the market which are offering the same leased-line services, including larger rival BT Openreach, would keep competition healthy.