The figures show an increase of roughly 17 per cent on 2014 when around 3.2 million people switched.
There were approximately 340,000 switches in December, just above the monthly average. Small and mid-suppliers made a net gain of more than 100,000 over the month.
The busiest month for switching in 2015 was October when almost 440,000 people changed supplier. The quietest was January when slightly more than 210,000 people switched.
Responding to the figures Lawrence Slade, chief executive of Energy UK, said: “Industry and government have made strides towards making switching simpler and quicker. Customers have become more engaged in switching and are shopping around to find the best deal for them.”
The secretary of state for energy and climate change, Amber Rudd, said: “Government has provided more choice in the energy market and there are now 34 energy suppliers to choose from, which is significantly more than in 2010.”