The company works with shops including Bartle Bogle Hegarty, Isobar, Joint and Adam & Eve/DDB. A Google spokesman said it was undertaking a standard review "to make sure that we are working with the right agencies and keep developing ideas for Google".
It is understood that the review started earlier this year, with agencies invited to tender their prices to Google.
Although the process is ongoing, Google has begun to contact agencies to tell them whether they have been successful in joining the roster.
Joint created Google’s "far from home" campaign for its Nexus 7 tablet that aired at Christmas. Google also ran a series of Christmas ads created by Adam & Eve/DDB that documented the most searched-for terms of 2013.
OMD UK, which handles Google’s UK media planning and buying, is unaffected by the review.