BT is under investigation by the telecoms regulator over allegations that it may have overcharged businesses when installing high-speed lines into offices.

Ofcom has launched a probe into the UK’s biggest telecoms firm after saying information provided by BT could have breached rules meant to protect small firms from facing large, up-front costs.

New rules introduced in May 2014 mean BT must absorb the first £2800 of excess construction costs associated with installing cables and technology into businesses signing up to BT’s leased line services.

Leased lines are high-speed, high-quality, point-to-point data connections that telecoms providers use for connecting offices, mobile base stations and broadband access networks, according to Ofcom.

The watchdog said: “Excess construction charges (ECCs) are levied by BT to recover the costs of customer-specific network construction required to extend BT’s existing network out to where the customer requires the new connection.”

Ofcom added: “BT has provided Ofcom with information indicating that it may not have correctly applied the ECC exemption to a number of relevant business connectivity orders since the beginning of the ECC exemption regime.

“Having considered the information provided by BT, we have decided to open an investigation.”