The energy giant was required to install around 22,400 advanced electricity meters to medium-sized business customers by 6 April 2014.

But it only managed to install the advanced meters at 15,200 of these meter points by deadline.

Ofgem also found that Npower didn’t take all reasonable steps to install advanced meters at 4,000 of these meter points, meaning it failed to meet legal requirements for the roll-out as part of its licence conditions.

As such, customers were left without an advanced meter, so they missed out on the opportunity to receive better information about their energy consumption and to control costs.

Npower was also found to have breached a second licence condition, as between October 2009 and November 2015, it replaced almost 200 electricity meters at relevant business premises with non-advanced meters.

Ofgem did seek to fine Npower £3.7m but it contested this, and a fine of £2.4m has been imposed instead.

Rob Salter-Church, Ofgem interim executive director for consumers and markets, said: “Npower is paying the price for failing to meet its obligations and letting down its business customers.

“The government set a clear deadline for suppliers to ensure no business customers unnecessarily missed out on the benefits of advanced meters, including the opportunity to save money on their bills. The fine reflects that there were systemic failings by Npower which led to the serious failure of not meeting the deadline.

“Suppliers must learn their lesson to make sure mistakes like this are not repeated in the roll-out of smart meters.”

An Npower spokesperson, said: “Ofgem’s announcement today draws a line under an investigation which relates to events several years ago. Npower remains committed to supporting our customers to benefit from AMR and SMART metering.”

Related: See YourMoney.com’s Smart meters under attack: what you need to know for more information.