Users of domestic smart devices have been warned that their data is being gathered even when the devices are not in use and passed on to third parties, enabling them to send targeted advertising.

The information was highlighted in two studies, one conducted by Imperial College London and Northeastern University in Boston, Massachusetts, and another from Princeton University.

The first found that devices including smart TVs made by companies such as LG and Samsung were sending data to companies including Netflix. The Princeton study found apps supported by US digital player Roku and FireTV were sending information to Google and others.

"Amazon is contacted by almost half the devices in our tests, which stands out because (this means) Amazon can infer a lot of information about what you're doing with different devices in your home, including those they don't manufacture," said one of the Northeastern paper's authors, David Choffnes.

The fact that much of the data sent is encrypted means authors could not identify precisely what was being shared."What they can exactly see depends on what the manufacturer is sending, which we have not made an attempt to re-engineer," said another of the paper's co-authors, Hamed Haddadi, from Imperial College.

The Imperial and Northeastern study looked at 81 different smart devices in the United Kingdom and United States, where a recent survey revealed around 68 percent of households have a connected TV device. The research found "notable cases of information exposure", with Amazon, Google, Microsoft and cloud service provider Akamai being the most frequently contacted companies.

Experts say the storage and sharing of data is barely regulated. "The situation is dire," Oxford University smart device scientist Max Van Kleek told the Financial Times. "People are spending more and more time on these devices, and they are placed in such critical places in people's homes, so we need to hold them to account."

Netflix insisted customers had nothing to worry about."Information Netflix receives from smart TVs that are not signed in is confined to how Netflix performs and appears on screen," said a statement from the company. "We do not receive any information about other applications or activity on smart TVs."

Google defended its involvement, saying: "Like other publishers, smart TV app developers can use Google's ad services to show ads against their content or measure the performance of ads. Depending on the user's chosen preferences on the device and consents, the publisher may share data with Google's that's similar to data used for ads in apps or on the web.

"Depending on the device manufacturer or the app owner, data sent to Google could include user location, device type and what the user is watching within a specific app so they can be targeted with personalized advertising."