MILAN, Sept 20 (Reuters) - British utility Centrica on Thursday signed a 2.4 billion cubic metre, three-year gas supply deal with the UK arm of Russian export monopoly Gazprom , it said in a statement. Gas deliveries will begin from 2014 and will be entirely priced against the UK's freely traded spot gas market, the National Balancing Point (NBP), Centrica said in a statement. The deal marks Gazprom's longest ever gas supply contract that is not linked to the price of oil, a Gazprom source told Reuters. Gazprom Marketing and Trading, the UK arm of the Russian parent, will deliver gas to Britain through a sub-sea pipeline from Belgium, the source added. The practice of linking supplies of gas to the price of oil has hurt Gazprom's European customers over the past few years because they are forced to sell the gas at a loss, costing them billions of euros in the process. The backlash against oil-linked contracts has gained ground in recent years as waning demand for gas and economic recession across Europe forces utilities to defend dwindling profit margins. In response to growing criticism over oil-indexation, the European Commission last week launched an investigation into suspected anti-competitive practices by Gazprom, adding to tensions between Europe and Moscow over energy policy. "Centrica is at the forefront of helping to deliver UK energy security and this deal is a natural fit with one of the world's largest gas producers and exporters," Cordi O'Hara, Centrica Energy origination director, said in a statement. The deal with Gazprom follows a string of supply deals for Norwegian gas and Qatari liquefied natural gas (LNG) last year. Centrica clinched a 10-year, 50 billion cubic metre supply deal with Norway's Statoil in November 2011, starting in 2015, and a major LNG contract with Qatar in February of the same year.