Google said the new set-up would enable it to tailor search results better.
But data regulators in France have cast doubt on the legality of the move and launched a Europe-wide investigation.
Google has merged 60 guidelines for its individual sites into a single policy for all of its services.
France’s privacy watchdog CNIL wrote to Google earlier this week, urging a “pause” in rolling out the revised policy.
“The CNIL and EU data authorities are deeply concerned about the combination of personal data across services,” the regulator wrote.
“They have strong doubts about the lawfulness and fairness of such processing, and its compliance with European data protection legislation.”
The regulator said it would send Google questions on the changes by mid-March.
In response, Google’s global privacy counsel Peter Fleischer said he was happy to answer any concerns CNIL had.
“As we’ve said several times over the past week, while our privacy policies will change on 1st March, our commitment to our privacy principles is as strong as ever,” Peter Fleischer wrote in a blog post.
Google rejected the regulator’s request to hold off on making the changes. Users are being moved on to the new single policy shortly after midnight on 1 March, local time.
Google’s business model – the selling of ads targeted on individual user behavior – relies on collecting browsing information from its visitors.
Until today, this information was kept apart between services.
This meant a search on, for example, YouTube, would have no significance on what results or advertising you would encounter on another Google site like Gmail.
The new agreement, which users cannot opt out of unless they stop using Google’s services, will mean activity on all of the company’s sites will be linked.
Many websites and blogs in the technology community have given guidance for users concerned about how their browsing history will be used.
They suggest users can access, and delete, their browsing and search history on the site by logging in to google.com/history.
A similar page for YouTube viewing and search history can also be accessed.
Users can see which Google services hold data about them by viewing their dashboard.
In preparation for the policy change, Google displayed prominent messages notifying visitors about the plans. A dedicated section was set up to provide more details.
However, campaign group Big Brother Watch has argued that not enough has been done to ensure people are fully aware of the alterations.
A poll of more than 2,000 people conducted by the group in conjunction with YouGov suggested 47% of Google users in the UK were not aware policy changes were taking place.
Only 12% of British Google users, Big Brother Watch said, had read the new agreement.
The group’s director Nick Pickles said: “If people don’t understand what is happening to their personal information, how can they make an informed choice about using a service?
“Google is putting advertisers’ interests before user privacy and should not be rushing ahead before the public understand what the changes will mean.”