Britain's competition regulator has launched an investigation into the power wielded by Facebook and Google in digital advertising markets, including the ownership of data.
The Competition and Markets Authority said it would look at how much influence the platforms had, the way they collected and used personal data and whether consumers were getting a good deal.
Britain last year hired Jason Furman, chief economist in former U.S. President Barack Obama's administration, to chair a new panel that reviewed the country's approach to digital technology, finding that tech giants needed more competition.
The CMA said if it finds evidence that there are problems, it could make detailed recommendations to government.
"Much about these fast-changing markets is a closed book to most people," CMA Chairman Andrew Tyrie said. "The work we do will open them up to greater scrutiny, and should give Parliament and the public a better grip on what global online platforms are doing.
"These are global markets, so we should and will work more closely than before with authorities around the world, as we all consider new approaches to the challenges posed by them."
(Reporting by Kate Holton, Editing by Paul Sandle)