Everything Everywhere (EE) will launch superfast mobile internet in October, according to Sky sources, which would make it the first UK provider to offer the network.
Telecoms regulator Ofcom approved an application by the company that owns Orange and T-Mobile to use its existing spectrum to deliver 4G.
But EE's rivals, who have to wait until the end of this year to buy new licences in an auction of frequencies, say the decision gives the company an unfair advantage.
A well-placed source at EE told Sky News the next generation of mobile broadband would be available in October, when EE launches a new, 4G-specific brand.
The source said the company plans to start dropping the Orange and T-Mobile brands from around March next year, with existing customers being migrated to EE.
The company applied for two trademarks - 4G Everything Everywhere@ and 4GEE@ - in May.
In response, an EE spokesperson said: "Everything Everywhere confirms that we are planning to launch a new brand in the UK later this year.
"This new brand will sit alongside our existing brands Orange and T-Mobile. We will reveal more information on our exciting plans in due course."
Ofcom gave EE the go-ahead to launch 4G - which is seen as crucial for video, gaming and downloading on your phone - any time after September 11 this year.
It said that allowing EE to use its existing 1800 MHz spectrum in this way will "deliver significant benefits to consumers".
"There is no material risk that those benefits will be outweighed by a distortion of competition," an Ofcom statement said.
"Delaying doing so would therefore be to the detriment of consumers."
But rival mobile company Vodafone said it was "frankly shocked" by the decision.
"The regulator has shown a careless disregard for the best interests of consumers, businesses and the wider economy through its refusal to properly regard the competitive distortion created by allowing one operator to run services before the ground has been laid for a fully competitive 4G market," a company statement said.
And mobile company Three agreed that the ruling could make the market less competitive.
"Liberalisation of 2G spectrum to date has distorted the competitive landscape in the UK, which ultimately harms consumers," Three said.
"Further liberalisation without addressing competition issues could make that distortion worse."
After the fourth generation auction at the end of this year, at least 98% of people across the UK should have access to the network.