But the move is likely to prompt investigation by UK regulators as the number of providers has now sunk to just three companies.
BT is attempting to buy EE but also has spectrum it can use itself. It isn’t yet ready to offer handsets itself, just SIM cards.
BT formerly owned O2, but sold it to Spanish telco Telefonica in 2005.
The company is likely to offer its sports service to people who sign up to its tariff, and that may be attractive to some people.
Paolo Pescatore, a director at CSS isnight, described BT Mobile’s launch as “more aggressive than many anticipated. He said: “BT has made the right decision to offer a range of of simple and transparent packages as part of its return to the consumer mobile market. The £5 SIM only deal for existing BT broadband households is probably the best value 4G SIM only deal in the market.”
He said BT is able to bring something different to the party by offering BT Sport and BT Wi-Fi.