Vodafone is in talks with BT’s network subsidiary Openreach about a groundbreaking joint investment in new ultrafast fibre-optic broadband for British cities.

The two companies are in what are described by industry sources as “early but serious” discussions about combining their financial strength to build large-scale new infrastructure to replace ageing copper telephone lines.

It is understood Vodafone plans to target the upgrades at major metropolitan areas initially, to allow it to provide faster and more reliable broadband to swathes of homes and businesses quickly.

The proposed joint investment has uncertain costs, with the price of new lines falling and under negotiation, but could run into billions of pounds over time. It would signal a radical shift in Britain’s telecoms industry.