Britain's advertising watchdog is being taken to the High Court over claims its lax definition of “fibre” broadband means customers are being hoodwinked into buying slow internet packages.

CityFibre, an Aim-listed ultra-fast broadband infrastructure builder, has filed a judicial review against the Advertising Standards Authority (ASA).

It is challenging an ASA ruling allowing use of the term “fibre” to describe services delivered over copper-based networks.

CityFibre argues the term should only apply to faster full fibre services. The ASA came under pressure from politicians and industry to review its definition of the term “fibre” last year, but ruled in November no change was required.

CityFibre’s lawyers will argue the ASA’s decision was “fundamentally flawed” and encourages internet service providers to “continue to mislead consumers”.

“You could hardly expect an automotive manufacturer to get away with advertising an ‘electric car’ when the most electric part of the car was its windows,” said Greg Mesch, chief executive of CityFibre. “The time has come to do away with ‘fake fibre’.”