The UK communications regulator Ofcom levied the fine for a “serious breach” of rules, according to a statement on Sunday.
The fine dwarfed any previous Ofcom penalty and puts another black mark on the record of chief executive, Gavin Patterson.
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The rebuke is a further blow to BT over its network division Openreach, which the company agreed to legally separate after months of difficult negotiations with the regulator that is seeking greater independence for the unit and more spending on full-fibre broadband.
The penalty tops a £3.7 million fine for the Vodafone Group last year for poor customer service.