Britain’s biggest mobile operators face explosive claims of attempted price fixing and collusion as part of a £1bn High Court battle over the sudden collapse of Phones 4U.

The former chief executive of the mobile operator O2 made “inappropriate” attempts to discuss pricing and smartphone sales plans with a direct rival, it is claimed.

At a crucial time for the mobile industry as it prepared for the shift from 3G to 4G networks, the former bosses of Vodafone UK and Carphone Warehouse allegedly sought to suggest higher pricing among operators.

The claims, which are denied by O2 are contained in documents filed at the High Court by EE, the mobile operator owned by BT. Along with O2 and Vodafone, it is defending itself against allegations by Phones 4U’s administrator that mobile operators unlawfully colluded to force the retailer out of business in 2014.

EE alleges that two years earlier former O2 chief executive Ronan Dunne, who is now a senior executive at the US telecoms giant Verizon, inappropriately sought to discuss pricing and sales plans with Olaf Swantee, its chief executive at the time.

Over lunch it is claimed Mr Dunne suggested O2 was willing to reduce smartphone supplies to independent retailers but wanted to “mitigate the risk of EE taking up that volume”.