One of the biggest makers of smart televisions has been found to be tracking users' viewing habits without them knowing.

Vizio has been fined $2.2 million (£1.8m) after the US consumer watchdog discovered the company had been using content recognition software to track viewers without asking for permission.

The tracking technology, called automated content recognition, can recognise what is being watched on the television at any given moment. Vizio gathered "as many as 100 billion data points a day from millions of TVs".

Vizio, which has sold more than 11 million smart TVs since 2010, was found to have been sharing the "mountain of data" with independent companies such as advertisers and those that monitor audience engagement and habits. It does not sell its TVs in the UK.

"Consumers didn't know that while they were watching their TVs, Vizio was watching them," the US Federal Trade Commission said. "The generic way the company described that feature – for example, 'enables program offers and suggestions' – didn’t give consumers the necessary heads-up to know that Vizio was tracking their TV’s every flicker."